Regional Water Table Research Articles

The ecohydrological imprint of deforestation in the semiarid Chaco: insights from the last forest remnants of a highly cultivated landscape

AbstractThe semiarid Chaco plains present one of the highest rates of forest clearing and agricultural expansion of the world. In other semiarid plains, such massive vegetation replacements initiated a groundwater recharge and salt mobilization process that, after decades, raised regional water tables and salts to the surface, degrading agricultural and natural ecosystems. Indirect evidence suggests that this process (known as dryland salinity) began in the Chaco plains. Multiple approaches (deep soil profiles, geoelectric surveys and monitoring of groundwater salinity, level and isotopic composition) were combined to assess the dryland salinity status in one of the oldest and most active agricultural hotspots of the region, where isolated forest remnants occupy an extremely flat cultivated matrix. Full vadose moisture and chloride profiles from paired agriculture‐forest stands (17 profiles, six sites) revealed the following: a generalized onset of deep drainage with cultivation (32 to >87 mm year−1), full leaching of native chloride pools (13.7 ± 2.5 kg m−2) down to the water table after >40 years following clearing and differential groundwater table rises (0.7 to 2 m shallower water tables under agriculture than under neighbouring forests). Continuous level monitoring showed abrupt water table rises under annual crops (up to 2.6 m in 15 days) not seen under forests or pastures. Varying deep drainage rates and groundwater isotopic composition under agricultural plots suggest that these pulses are strongly modulated by crop choices and sequences. In contrast to other dryland salinity‐affected areas of the world, forest remnants in the study area (10–20% of the area) are not only surviving the observed hydrological shifts but also sustaining active salty groundwater transpirative discharge, as evidenced by continuous water table records. The overall impact of these forest remnants on lowering neighbouring water tables would be limited by the low hydraulic conductivity of the sediments. As highly cultivated areas of the Chaco evolve to new hydrological conditions of shallower saline water tables, innovative crop rotations that minimize recharge, enhance transpirative discharge and tolerate salinity will be needed. Copyright © 2016 John Wiley & Sons, Ltd.

Genesis, distribution and dynamics of lagoon marl extrusions along the Curonian Spit, southeast Baltic Coast

A unique geological process of extrusion of lagoon marl from beneath massive migrating sand dunes is characteristic for large segments of the Curonian Spit – a ~100‐km‐long sandy barrier that separates the Curonian Lagoon from the Baltic Sea. Exposures of a composite set of Holocene organic sediments such as gyttja, clayey gyttja and gyttja clay, collectively referred to as ‘lagoon marl’, are common along the northern half of the lagoon coast of the spit. The marl outcrops rise up to 3–4 m above the lagoon level and owe their origin to extrusion from their 7–8 m in situ depth beneath the present regional water table. New detailed investigations of the Baltic Sea bottom along the southern half of the Curonian Spit using side‐scan sonar, a multibeam echosounder, seismic imaging, sediment sampling and video observations allowed identification and mapping of a unique underwater landscape formed by extensive outcrops of laminated and folded lagoon marl at water depths of 5–15 m. The combined onshore–offshore database indicates that relict lagoon marl was deformed, compacted and dehydrated by a massive dune‐covered coastal barrier migrating landward (retrograding) over these sediments during the Littorina Sea transgression in a processes termed ‘dune tectonics’. Spatial analysis of the relict sediments traced in offshore geophysical data helped to constrain the rates of the southeast migration of the dune massif. A conceptual model is presented to explain the present context of marl exposures above the regional water table, as well as the occurrence of relict lagoon marl extrusions (diapirs) on the underwater marine slope of the Curonian Spit.

Modeling of transient water table response to managed aquifer recharge: a lagoon in Muscat, Oman

Natural and anthropogenic conditions in the Gulf countries lead to substantial changes in groundwater (GW) dynamics. GW mound under a lagoon supplied by treated wastewater (TWW) from a municipal sewage treatment plant (STP) in Muscat, Oman, is studied using analytical and numerical modeling techniques. The water table mound in an unconfined aquifer, laterally bounded by the sea, receives recharge from a lagoon fed by a STP in a seasonally varying regime. The onset and duration of the filling period and, correspondingly, infiltration from the lagoon bed is dictated by the wastewater processing and TWW use. Following the period of recharge-induced growth, the GW mound decays when recharge ceases, and the void space above the water table becomes available for future injection in the next annual cycle of infiltration and TWW subsurface storage. The Green-Ampt model application illustrates that the first phase of the process (vertical infiltration with front propagating from the lagoon bed to the regional flat water table) is typically short (several hours). The second phase, when TWW gets in a direct hydraulic contact with GW, is the longest and with a mound spreading to hundreds of meters during winter months. No analytical solutions to this moving boundary-value problem are known and approximate models, based on the Boussinesq approximation and its linearizations, are utilized. The Hantush-type drawup of the water table in the vicinity of the lagoon is studied by the Watson analytical solution to the diffusion equation for a vertically averaged hydraulic head in an unbounded domain, with an instantaneous jump of the head over a disk-shaped zone. This drawup is compared with MODFLOW simulations, which take into account a finite size of the flow domain (catchment) and discharge into the sea through a constant-head segment. The Mishra-Guyonnet’s formula assumes a spatially uniform flux from the lagoon bed (disk in a model plain) into a growing mound and is used for calculation of the total discharge, which progressively decreases from the early stages of the Green-Ampt infiltration to a theoretically zero value if the second phase lasts indefinitely long. We assess characteristic times and phases of the mound dynamics during the rise and fall stages using regional hydrogeological parameters, site geometry, and typical patterns of wastewater disposal. Sensitivity analysis of the locus of the phreatic surface at a given instance and location is carried out, with storativity, hydraulic conductivity, undisturbed aquifer thickness and lagoon radius as input variables. For an example of an aquifer of saturated depth of 40 m, undisturbed vadose zone thickness of 10 m, saturated hydraulic conductivity of 10 m/day, effective porosity of 0.2, a circular lagoon size of 90 m and water depth there of 1 m we found that the infiltration front propagates to the water table in less than 0.2 days; after 40 days of a continuous mound growth in a hypothetical piezometer located 400 m from the lagoon the water table drawup was in the range of 2.0–2.7 m if the aquifer hydraulic parameters and lagoon size are varied 20 % from the average value. This approach will permit to plan more detailed site investigation, facilitate management decisions on the rate of water release to the lagoon, and foresee waterlogged areas.

Variabilité spatiotemporelle sur le long terme des précipitations en zone aride en Australie et conséquences sur la recharge

Quantifying dryland groundwater recharge as a function of climate variability is becoming increasingly important in the face of a globally depleted resource, yet this remains a major challenge due to lack of adequate monitoring and the complexity of processes involved. A previously unpublished and unique dataset of high density and frequency rainfall measurements is presented, from the Fowlers Gap Arid Zone Research Station in western New South Wales (Australia). The dataset confirms extreme spatial and temporal variability in rainfall distribution which has been observed in other dryland areas and is generally explained by the dominance of individual storm cells. Contrary to previous observations, however, this dataset contains only a few localised storm cells despite the variability. The implications of spatiotemporal rainfall variability on the estimation of groundwater recharge is assessed and show that the most likely recharge mechanism is through indirect and localised, rather than direct, recharge. Examples of rainfall and stream gauge height illustrate runoff generation when a spatially averaged threshold of 15–25 mm (depending on the antecedent moisture conditions) is exceeded. Preliminary assessment of groundwater levels illustrates that the regional water table is much deeper than anticipated, especially considering the expected magnitude of indirect and localised recharge. A possible explanation is that pathways for indirect and localised recharge are inhibited by the large quantities of Aeolian dust observed at the site. Runoff readily occurs with water collecting in surface lakes which slowly dry and disappear. Assuming direct groundwater recharge under these conditions will significantly overestimate actual recharge.

Representing northern peatland microtopography and hydrology within the Community Land Model

Abstract. Predictive understanding of northern peatland hydrology is a necessary precursor to understanding the fate of massive carbon stores in these systems under the influence of present and future climate change. Current models have begun to address microtopographic controls on peatland hydrology, but none have included a prognostic calculation of peatland water table depth for a vegetated wetland, independent of prescribed regional water tables. We introduce here a new configuration of the Community Land Model (CLM) which includes a fully prognostic water table calculation for a vegetated peatland. Our structural and process changes to CLM focus on modifications needed to represent the hydrologic cycle of bogs environment with perched water tables, as well as distinct hydrologic dynamics and vegetation communities of the raised hummock and sunken hollow microtopography characteristic of peatland bogs. The modified model was parameterized and independently evaluated against observations from an ombrotrophic raised-dome bog in northern Minnesota (S1-Bog), the site for the Spruce and Peatland Responses Under Climatic and Environmental Change experiment (SPRUCE). Simulated water table levels compared well with site-level observations. The new model predicts hydrologic changes in response to planned warming at the SPRUCE site. At present, standing water is commonly observed in bog hollows after large rainfall events during the growing season, but simulations suggest a sharp decrease in water table levels due to increased evapotranspiration under the most extreme warming level, nearly eliminating the occurrence of standing water in the growing season. Simulated soil energy balance was strongly influenced by reduced winter snowpack under warming simulations, with the warming influence on soil temperature partly offset by the loss of insulating snowpack in early and late winter. The new model provides improved predictive capacity for seasonal hydrological dynamics in northern peatlands, and provides a useful foundation for investigation of northern peatland carbon exchange.

Preliminary hydrogeophysical investigation at the 10th of Ramadan City, Egypt, by 1D and 2D inversion of VES data

ABSTRACTElectrical resistivity survey in the form of vertical electrical sounding was conducted at the 10th of Ramadan City, Egypt, to analyse subsurface lithology distribution and the inter‐relation between the possible infiltrated wastewater from oxidation ponds and the surface seepage emerged at the city centre. Special interest was paid for tracing the low‐resistivity clayey layer, which can act as a barrier against surface water percolation and as a low permeable zone to form local perched water system. Vertical electrical sounding data by Schlumberger electrode configuration were taken at 17 stations along a line extended from the city centre to the oxidation ponds used for wastewater treatment. The data were inverted by using 1D and 2D inversion schemes to construct geoelectrical cross section along the surveyed line. The geoelectrical layers were interpreted principally in terms of lithology and water content by correlation with borehole information. From the obtained results, four layers were identified in the subsurface medium. The upper three layers are located above the regional water table, whereas the fourth one was interpreted to be fully water saturated. The second layer from the surface is of great importance as it is the low‐resistivity clayey layer. This layer is discontinuous along the survey line. Downward infiltration of wastewater from the oxidation ponds is possible especially at the pond No. 2 as the clayey layer was interpreted not to exist at the site. The infiltrated wastewater from the oxidation ponds was interpreted not to be the cause of the surface seepage emerged at the city centre because the clayey layer dips toward opposite direction. The probable sources of the seepage were considered the local surface infiltration from excess irrigation water in the nearby cultivated land and/or leakage from the underground water utilities in the city centre.

Three dimensional characterization of complex mantled karst structures. Decision making and engineering solutions applied to a road overlying evaporite rocks in the Ebro Basin (Spain)

At mantle/cover evaporite karst settings, evidences of surficial subsidence are usually strongly dependent on geomechanical properties and spatial inhomogeneities within the cover series. In infrastructure assets, affected by karst processes, the evaluation of mitigation or engineering solutions requires a 3D reconstruction of the underground, karst-affected materials. Analysis and surveillance are also required to avoid deterioration of natural conditions and the migration or the increase in rate of karst features. In the central Ebro Basin, fluvial deposits cover soluble salts (mainly gypsum) located below the regional water table. Variations of geomechanical parameters of the cover, mainly related to carbonate crusts, and the contemporaneous sedimentation of fluvial deposits during karst activity can complicate the characterization of affected volumes. In this work, an integrated analysis including the available information, 3D characterization by means of GPR and three boreholes in a peri-urban road was carried out to finally define the geometry and evolution of karst phenomena. An evaluation of metastable situations is also considered in order to define the zoning for hazard mitigation. This evaluation shows that the volumetric affection to the cover series is in the range of tens of thousands of cubic meters. A proposed list of factors related to social aspects, infrastructure relevance and budget is analyzed.

Groundwater dynamics under water-saving irrigation and implications for sustainable water management in an oasis: Tarim River basin of western China

Abstract. Water is essential for life. Specifically in the oases of inland arid basins, water is a critically limited resource, essential for the development of the socio-economy and the sustainability of eco-environmental systems. Due to the unique hydrological regime present in arid oases, a moderate groundwater table is the goal of sustainable water management. A shallow water table induces serious secondary salinization and collapse of agriculture, while a deep water table causes deterioration of natural vegetation. From the hydrological perspective, the exchange flux between the unsaturated vadose zone and groundwater reservoir is a critical link to understanding regional water table dynamics. This flux is substantially influenced by anthropogenic activities. In the Tarim River basin of western China, where agriculture consumes over 90% of available water resources, the exchange flux between the unsaturated vadose zone and groundwater reservoir is influenced strongly by irrigation. Recently, mulched drip irrigation, a sophisticated water-saving irrigation method, was widely applied in the Tarim River basin, which greatly impacted the exchange flux and thus the regional groundwater dynamics. Capitalizing on recent progress in evaporation measurement techniques, we can now close the water balance and directly quantify the exchange flux at the field scale, thus gaining a better understanding of regional groundwater dynamics. In this study, comprehensive observations of water balance components in an irrigated cropland were implemented in 2012 and 2013 in a typical oasis within the Tarim River basin. The water balance analysis showed that the exchange flux and groundwater dynamics were significantly altered by the application of water-saving irrigation. The exchange flux at the groundwater table is mostly downward (310.5 mm year−1), especially during drip irrigation period and spring flush period, while the upward flux is trivial (16.1 mm year−1) due to the moderate groundwater table depth (annual average depth 2.9 m). Traditional secondary salinization caused by intense phreatic evaporation (fed by upward exchange flux) is alleviated. However, a new form of secondary salinization may be introduced unwittingly if there is lack of water for periodic flushing, especially when brackish water is used in the irrigation. Furthermore, the water saved via drip irrigation has been used in further growth of irrigated lands instead of supporting the ecological system. This could lead to an increased risk of eco-environmental degradation and calls for improved governance schemes. The insights gained from this study can be potentially applied to other arid inland areas (e.g., central Asia) which face similar water shortages and human development problems.

Climate, palaeohydrology and land use change in the Central Iberian Range over the last 1.6 kyr: The La Parra Lake record

A multiproxy study of a 7 m long sedimentary sequence from Lake La Parra (39°50.948′, 1°52′, 1014 m a.s.l.) supported by 11 14C AMS and 210Pb/137Cs dates provides a robust, high-resolution hydrological and environmental variability record for the last 1600 years of the Las Torcas sinkhole Complex in the Central Iberian Range. The succession of depositional environments in Lake La Parra sinkhole is controlled by both changes in the regional water table and by the balance between sedimentary input through ephemeral creeks and in-lake production of carbonates and organic matter. Although synergetic links with climate are likely, phases of increased sediment delivery to the lake at c. ad 500–700, c. ad 1000, ad 1450–1500, ad 1550–650 and since 1700 till recent times are driven primarily by human impact in the watershed. Prior to c. ad 300, the sinkhole was dry, then became a lake at the end of the Roman Period (ad 350) when the doline was flooded, and it has not dried out during the last 1600 years. Moderate lake levels with deposition of coarser clastic facies dominated up to the 12th century (ad 400–1200), and relatively higher levels with deposition of laminated facies during the 13th–15th centuries (ad 1200–1600). The pattern of palaeohydrological evolution at a centennial scale is roughly coherent with most Iberian lacustrine records; however, the ‘La Parra’ sequence indicates that increased humidity during Iberian–Roman times was restricted to southern Spain and the humid phases of the ‘Little Ice Age’ (‘LIA’) starting and ending earlier in the central Iberian Range compared with the Pyrenean Domain and southern Spain. This new sequence highlights the heterogeneity through space and time of the main dry and wet climatic periods at shorter scales, emphasizing the impact of latitudinal climate gradients on the Iberian Peninsula climate variability.

Hydrogeologic insights for a Devil's Slide‐like system

AbstractTectonically active coastal margins commonly host landslides that are influenced by hydrologic, geologic, and/or anthropogenic perturbations. The work reported here is motivated by the hydrologically driven, deep‐seated bedrock slides that intersect the (former) Pacific Coast Highway in the active landslide zone at Devil's Slide near Pacifica, California. Numerical simulation of subsurface flow is employed to investigate saturated zone fluid pressure scenarios for 3‐D Devil's Slide‐like systems. The four‐phase concept‐development effort is comprised of 134 hydrogeologic simulation scenarios which investigate fluid pressure response for complex subsurface conditions and historically based climate forcings. Recharge, heterogeneity, and anisotropy are shown to increase fluid pressures in targeted failure‐prone locations by up to 73.8, 10.3, and 1.8 %, respectively. The interaction between fault zone characteristics and topographically driven flow are shown to influence fluid pressures for up to 85% of the approximately 7.0 105 m2 study area. Simulated fluid pressures support the known slope instability for the Devil's Slide site. A quantitative hypothesis‐testing discussion explores the likelihood of perched water above the regional water table at the site. Further understanding of hydrologically driven slope movement in the active landslide zone will require additional data focused on rigorous characterization of the unsaturated zone.

Holocene floodplain palaeoecology of the Humberhead Levels; implications for regional wetland development

The acquisition of high quality, well-dated local site records is essential for progressing regional environmental reconstructions. As part of a wider study designed to examine intra- and extra- site ecosystem responses to environmental change, this paper presents new palaeoecological data from the floodplain of the River Torne in the Humberhead Levels, South Yorkshire. The sampling site lies adjacent to the lowland raised mire of Hatfield Moors, a location with a long history of palaeoecological investigations. The potential of using floodplain records to reconstruct local variations in ecosystem response to environmental change is also considered. Coleoptera and pollen are used to reconstruct floodplain ecosystem dynamics, whilst chronologies are established using Bayesian age–depth modelling. Between 10,200 cal BP and 2300 cal BP, the floodplain experienced multiple phases of ecological change. At 10,200–9910 cal BP, a cut-off channel began to infill with peat, while the surrounding floodplain remained relatively dry with Pinus forest growing nearby. Between 9630–9500 cal BP and 7270–7020 cal BP, a depositional hiatus occurred in the sedimentary record. By the end of this period, the local woodland had diversified and expanded to mixed deciduous tree cover. A wet shift identified at 6870–6160 cal BP was shortly followed by a rise in Alnus and Tilia from 6410–6160 cal BP. At this time, widespread floodplain paludification had occurred in the Humberhead Levels, which was largely controlled by relative sea-level (RSL) rise and the associated rise in regional water tables. Floodplain expansion also resulted in the widespread occurrence of Alnus dominated fen woodland. The local Torne floodplain experienced varying levels of wetness that influenced the decline and subsequent regeneration of the woodland from 5870–5640 cal BP. At this time, the Ulmus decline is identified in the pollen stratigraphic record. Floodplain hydrology appears to have been controlled by a combination of water table fluctuations and changes in channel pattern/flow, both of which can be linked to RSL variations recorded in the Humber Estuary. Floodplain alluviation, also linked to rising water tables, is dated to 4360–4160 cal BP. Anthropogenic woodland clearance further upstream may have further compounded this event.

Climatic and hydrologic processes leading to wetland losses in Yellowstone National Park, USA

Summary Wetlands are vital habitats and can be used as landscape indicators because they integrate catchment-scale processes. Wetland drying during the recent decades in Yellowstone National Park’s Northern Range has incited concern among National Park managers and the public at large. Our research was focused on developing an understanding of the processes controlling wetland water levels and the changes contributing to wetland decline in the Northern Range. We integrated analyses of hydrology, climate, soils, and vegetation. In 2009, 24 study wetlands were instrumented each with an average of five shallow groundwater monitoring well and piezometer nests. We mapped hydric soils, analyzed aerial photographs, and identified geomorphic indicators of higher water to quantify historic wetland area. The Trumpeter Lake study site was intensively studied to resolve watershed processes driving water table changes through time, and it was used to identify the timescale on which a regionally critical wetland varies. Climate data indicated that warming and drying occurred during the last century, but that this pattern was within the natural range of variation for the study region over the past 800 years, as determined from tree ring data. Hydrologic data revealed that study sites included locations of groundwater discharge, recharge, and flow-through as well as water perched above the regional water table. Hydrologic regimes were classified using a shape-magnitude framework and seven wetland classes were characterized, and the robustness of this classification is assessed using longer-term datasets. Aerial photographs and hydric soil delineation both confirmed formerly greater wetland abundance. Changes varied by wetland class and the presence or absence of surface water outlets. Wetland plant species inhabited distinct habitats of water table depth and variation, and can be used to infer subsurface hydrologic regime in the absence of extensive monitoring well networks. A subset of long-term monitoring wells has been instrumented with groundwater pressure transducers, enabling further understanding of wetland hydrologic processes, promoting additional assessment of the wetland classification, and aiding in identification of wetlands especially vulnerable to climate change.

Radiocarbon Dating, Mineralogy, and Isotopic Composition of Hackberry Endocarps from the Neolithic Site of Aşikli Höyük, Central Turkey

Carbonate is abundant in many Neolithic tells and is a potentially useful archive for dating and climate reconstruction. In this paper, we focus on the mineralogy, radiocarbon dating, and stable isotope systematics of carbonate in hackberry endocarps. Hackberry fruits and seeds are edible in fresh and stored forms, and they were consumed in large quantities in many Neolithic sites in the Near East, including the site of our study, Aşıkli Höyük in central Anatolia, an Aceramic Neolithic tell occupied from about 9.4 to > 10.3 BP (7.4 to > 8.3 BCE). Detailed 14C age control provided by archaeological charcoal permits a test of the fidelity in 14C dating of hackberry endocarps. Modern endocarps and leaves yield fraction modern 14C values of 1.050–1.066, consistent with levels present in the atmosphere when sampled in 2009. On the other hand, archaeological endocarps yield consistently younger ages than associated charcoal by ca. 130 14C years (ca. 220 calendar years) for samples about 10,000 years old. We speculate this is caused by the slight addition of calcite or recrystallization to calcite in the endocarp, as detected by scanning electron microscopy. Subtle addition or replacement of calcite by primary aragonite is not widely recognized in the 14C community, even though similar effects are reported from other natural carbonates such as shell carbonate. This small (but consistent) level of contamination supports the usefulness of endocarps in dating where other materials like charcoal are lacking. Before dating, however, hackberries should be carefully screened for mineralogical preservation and context. We examined the carbon and oxygen isotopic systematics of the fossil endocarps to try to establish potential source areas for harvesting. Most of the hackberries are enriched in 18O compared to local water sources, indicating that they were drawing on highly evaporated soil water, rather than the local (perched and regional) water table sampled in our study. Isotopic evidence therefore suggests that most but not all of the hackberries were harvested from nearby mesas well above the local streams and seeps fed by the water table.

Radiocarbon Dating, Mineralogy, and Isotopic Composition of Hackberry Endocarps from the Neolithic Site of Aşikli Höyük, Central Turkey

Carbonate is abundant in many Neolithic tells and is a potentially useful archive for dating and climate reconstruction. In this paper, we focus on the mineralogy, radiocarbon dating, and stable isotope systematics of carbonate in hackberry endocarps. Hackberry fruits and seeds are edible in fresh and stored forms, and they were consumed in large quantities in many Neolithic sites in the Near East, including the site of our study, Aşıkli Höyük in central Anatolia, an Aceramic Neolithic tell occupied from about 9.4 to > 10.3 BP (7.4 to > 8.3 BCE). Detailed14C age control provided by archaeological charcoal permits a test of the fidelity in14C dating of hackberry endocarps. Modern endocarps and leaves yield fraction modern14C values of 1.050–1.066, consistent with levels present in the atmosphere when sampled in 2009. On the other hand, archaeological endocarps yield consistently younger ages than associated charcoal by ca. 13014C years (ca. 220 calendar years) for samples about 10,000 years old. We speculate this is caused by the slight addition of calcite or recrystallization to calcite in the endocarp, as detected by scanning electron microscopy. Subtle addition or replacement of calcite by primary aragonite is not widely recognized in the14C community, even though similar effects are reported from other natural carbonates such as shell carbonate. This small (but consistent) level of contamination supports the usefulness of endocarps in dating where other materials like charcoal are lacking. Before dating, however, hackberries should be carefully screened for mineralogical preservation and context. We examined the carbon and oxygen isotopic systematics of the fossil endocarps to try to establish potential source areas for harvesting. Most of the hackberries are enriched in18O compared to local water sources, indicating that they were drawing on highly evaporated soil water, rather than the local (perched and regional) water table sampled in our study. Isotopic evidence therefore suggests that most but not all of the hackberries were harvested from nearby mesas well above the local streams and seeps fed by the water table.

When Can Inverted Water Tables Occur Beneath Streams?

Decline in regional water tables (RWT) can cause losing streams to disconnect from underlying aquifers. When this occurs, an inverted water table (IWT) will develop beneath the stream, and an unsaturated zone will be present between the IWT and the RWT. The IWT marks the base of the saturated zone beneath the stream. Although a few prior studies have suggested the likelihood of an IWT without a clogging layer, most of them have assumed that a low-permeability streambed is required to reduce infiltration from surface water to groundwater, and that the IWT only occurs at the bottom of the low-permeability layer. In this study, we use numerical simulations to show that the development of an IWT beneath an unclogged stream is theoretically possible under steady-state conditions. For a stream width of 1 m above a homogeneous and isotropic sand aquifer with a 47 m deep RWT (measured in an observation point 20 m away from the center of the stream), an IWT will occur provided that the stream depth is less than a critical value of 4.1 m. This critical stream depth is the maximum water depth in the stream to maintain the occurrence of an IWT. The critical stream depth decreases with stream width. For a stream width of 6 m, the critical stream depth is only 1 mm. Thus while theoretically possible, an IWT is unlikely to occur at steady state without a clogging layer, unless a stream is very narrow or shallow and the RWT is very deep.

Transient hydrological conditions implied by chloride mass balance in southeast Australian rivers

A robust correlation between electrical conductivity (EC) values and Cl concentrations in river water from southeast Australia allows detailed Cl fluxes to be calculated from continuous EC and river discharge records. Many Victorian rivers export significantly more Cl than is delivered to their catchments by rainfall. Cl* is defined as the mass of Cl exported in the rivers relative to that input by rainfall over a multi-year period (Cl*=100% indicates that the river exports the same mass of Cl as is input by rainfall). There is a systematic relationship between catchment type and Cl*. Rivers draining cleared plains have Cl* values between 50 and 750%, rivers draining volcanic plains have Cl* values of 770–1600%, whereas rivers with large forested upland catchments have Cl* values of 50–110%. These values are minima as they do not account for Cl exported by groundwater from the catchments. The calculations are based on long-term (up to 22year) records that span drought and high rainfall periods. The magnitude of Cl* is far higher than can be explained by errors in the calculations or variability in rainfall and runoff, and Cl/Br ratios preclude halite dissolution as a source of Cl. The excess Cl reflects hydrological changes in the catchments. Land clearing on the cleared plains has caused the rise of regional water tables which results in the export of Cl from saline groundwater via increased baseflow to the river systems. Drainage systems on the volcanic plains are re-establishing following impoundment by recent (<4.5Ma) lava flows; Cl which accumulated in shallow groundwater around saline lakes and marshes developed on these volcanic plains is now being exported via the rivers. The upland catchments have undergone less landscape change and may be in chemical balance. The methodology outlined here provides a straightforward assessment of whether catchments are in chemical balance that may in turn indicate whether they are undergoing hydrological changes.

Field assessment of surface water–groundwater connectivity in a semi‐arid river basin (Murray–Darling, Australia)

AbstractIn semi‐arid and arid river basins, understanding the connectivity between rivers and alluvial aquifers is one of the key challenges for the management of groundwater resources. The type of connection present (gaining, losing‐connected, transitional and losing‐disconnected) was assessed at 12 sites along six Murray–Darling Basin river reaches. The assessments were made by measuring the hydraulic head in the riparian zone near the rivers to evaluate if the water tables intersected the riverbeds and by measuring fluid pressure (ψ) in the riverbeds. The rationale for the latter was that ψ will always be greater than or equal to zero under connected conditions (either losing or gaining) and always lesser than or equal to zero under losing‐disconnected conditions. A mixture of losing‐disconnected, losing‐connected and gaining conditions was found among the 12 sites. The losing‐disconnected sites all had a riverbed with a lower hydraulic conductivity than the underlying aquifer, usually in the form of a silty clay or clay unit 0.5–2 m in thickness. The riparian water tables were 6 to 25 m below riverbed level at the losing‐disconnected sites but never lower than 1 m below riverbed level at the losing‐connected ones. The contrast in water table depth between connected and disconnected sites was attributed to the conditions at the time of the study, when a severe regional drought had generated a widespread decline in regional water tables. This decline was apparently compensated near losing‐connected rivers by increased infiltration rates, while the decline could not be compensated at the losing‐disconnected rivers because the infiltration rates were already maximal there. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Airborne TEM for the recovery of basin scale solute distribution; Perth Basin, Western Australia.

The distribution of groundwater salinity is a key input for management of water resources. Estimates of the three dimensional distribution of groundwater salinity below areas spanning thousands of square kilometres may be required. Airborne transient electromagnetic methods provide the possibility of recovering first pass large scale solute concentration distributions provided lithological influences on electrical conductivity distribution are not dominant. The Allanooka airborne TEM survey is located in the northern most portion of the Perth Basin in Western Australia. We use data from this Airborne TEM survey combined with data recovered from monitoring wells to highlight the steps used to construct a first pass large scale solute distribution model. We provide a method for converting airborne TEM datasets to estimates of solute concentration distribution for sandstone dominated sediments at a basin scale. For the Allanooka monitoring well network, base line empirical relationships are developed between laboratory derived total dissolved solids and formation conductivity derived from wire line logs. This relationship is then extended to include airborne TEM derived formation conductivities. Appropriate layer discretisation of input seed models for inversion of the airborne TEM data set are based on analysis of resistivities derived from wire-line logs. The interpretation of the inverted airborne TEM was assisted by geological constraints and high resolution seismic reflection transects. Selected inversion statistics were also mapped throughout the 3D volume to provide a quick method for assessing the ?importance? of particular layers to the outcome of the inversion. An approximate volume of low solute concentration sandstone dominated formation below the regional water table was extracted from the airborne TEM data. Our first pass basin-scale Airborne TEM derived 3D solute concentration provides a starting point for more detailed interpretation to commence.

Changing Climate and Sea Level Alter Hg Mobility at Lake Tulane, Florida, U.S.

Between 45,000 cal years BP and the beginning of the Holocene, the accumulation rate for Hg in sediments of Lake Tulane, Florida ranged from ≈2 to 10 μg m(-2) yr(-1), compared with 53 μg Hg m(-2) yr(-1) in the 1985-1990 period of anthropogenic input. The locality experienced regional draw-down of the water table during the Wisconsinan glaciation, which lowered global sea level by nearly 130 m. Natural atmospheric deposition of Hg to the surrounding area resulted in long-term (ca. 100,000 years) sequestration of this atmospheric flux of Hg, primarily by adsorption in the oxic Al- and Fe-hydroxide-rich sandy subsoil. Global sea level rise during deglaciation led to a rising regional water table, flooding the oxidized soils surrounding Tulane. Iron and adsorbed Hg were mobilized by reductive dissolution and transported by groundwater flow to Lake Tulane and ultimately to the accumulating sediment. The accumulation rate of Hg (and Fe) increased rapidly about 16,000 cal years BP, peaked at nearly 60 μg Hg m(-2) yr(-1) ca. 13,000-14,000 cal years BP, declined sharply during the Younger Dryas, and then increased sharply to a second 60 μg Hg m(-2) yr(-1) peak about 5000 cal years BP. Thereafter, it declined nearly to background by 900 cal years BP. In similar geologic situations, rapid modern sea level rise will initiate this process globally, and may mobilize large accumulations of Hg and lesser amounts of As, and other redox sensitive metals to groundwater and surface water.

Methane fluxes from a patterned fen of the northeastern part of the La Grande river watershed, James Bay, Canada

In northeastern Canada, at the ecotonal limit of the forest tundra and lichen woodland, a rise of the regional water table in the peatland systems was registered since Little Ice Age resulting in increasing pool compartment at the expense of terrestrial surfaces. We hypothesized that, with a mean water table closer to peat surface and higher pool density, these ecosystems would be great CH4 emitters. In summers 2009 and 2010, methane fluxes were measured in a patterned fen located in the northeastern portion of the La Grande river watershed to determine the contribution of the different microforms (lawns, hollows, hummocks, string, pools) to the annual CH4 budget. Mean seasonal CH4 fluxes from terrestrial microforms ranged between 12.9 and 49.4 mg m−2 day−1 in 2009 and 15.4 and 47.3 mg m−2 day−1 in 2010. Pool fluxes (which do not include ebullition fluxes) ranged between 102.6 and 197.6 mg CH4 m−2 day−1 in 2009 and 76.5 and 188.1 mg CH4 m−2 day−1 in 2010. Highest fluxes were measured in microforms with water table closer to peat surface but no significant relationship was observed between water table depth and CH4 fluxes. Spatially weighted CH4 budget demonstrates that, during the growing season, the studied peatland emitted 66 ± 31 in 2009 and 55 ± 26 mg CH4 m−2 day−1 in 2010, 79 % of which is accounted by pool fluxes. In a context where climate projections predict greater precipitations in northeastern Canada, these results indicate that this type of peatlands could contribute to modify the methane balance in the atmosphere.