Local Groundwater Table Research Articles

Analyzing Coastal Groundwater Variability in Sindhudurg, Maharashtra: A Spatio-temporal GIS Approach+6

Due to a sharp increase in groundwater demands in coastal regions, groundwater levels are getting depleted day by day. The local groundwater table fluctuation eventually speeds up sea water intrusion. The present study investigates spatial and temporal groundwater level fluctuations to identify the area affected by sea water intrusion as well as for sustainable water resource management in coastal stretch of Sindhudurg district, Maharashtra, which is about 121 km. Using groundwater level data collected from Central Groundwater Board (CGWB), Sindhudurg for the period from 1991 to 2022, fluctuations of groundwater levels were examined. The collected levels were evaluated for monsoon, pre-monsoon & post-monsoon seasons and all levels were interpolated over the study area using the Inverse Distance Weighted Method as a spatial interpolation technique in QGIS 3.22.1, in order to obtain the actual differences in groundwater levels. According to the results, the groundwater level varies from 1.88m to 13.45 m below ground level (bgl) in the pre-monsoon season, from 0.1m to 7.05 m (bgl) in the post-monsoon season and from 1m to 10.6m (bgl) in the monsoon season. Using statistical tests like Mann-Kendall test & Sen's slope estimator, the pattern of variation of groundwater levels was anticipated. According to our findings, over 80% of the basin has a falling water table during the monsoon season. It was observed that the groundwater level in the pre-monsoon season was more as compared to those in the monsoon and the post-monsoon seasons in the entire study area. The overall outcome of the study demonstrates that the groundwater level fluctuations were non-uniform in the study area, which may cause an increase in the sea water intrusion, leading to a decrease in crop production. Keywords: GIS, Groundwater level, Sea water intrusion, Mann Kendall test, Spatial interpolation, Sen’s slope estimator.

An analytical solution of the tide-induced groundwater table overheight under a three-dimensional kinematic boundary condition

Time averaged groundwater table above the mean sea level has been observed in unconfined coastal aquifers. Several theoretical solutions were presented to investigate the groundwater table overheight (super-elevation) in response to tidal oscillations. Nevertheless, previous analytical solutions are generally limited to Dupuit-Forchheimer assumptions or a two-dimensional free surface model. In this paper, we introduced a more general condition that a three-dimensional kinematic boundary condition is considered. An exact analytical solution of the groundwater table overheight under the multi-tidal constituents was derived using the Fourier transform method. The solution shows the local groundwater table overheight is related to the mean aquifer thickness, tidal amplitude, damping coefficient as well as its positions in sand beaches. In addition, the analytical solution illustrates the simple linear superposition is not applicable to the multi-tidal components condition, whose groundwater table overheight becomes weak due to the non-linear effect of the three-dimensional kinematic boundary condition. In addition, concentrations were focused on the time averaged total flux density illustrating the groundwater exchange occurs both in the shore normal and shore parallel directions, which however is missing in the previous two-dimensional solution. Further cognitions on the groundwater circulation mechanism were also achieved, which originates from the large tidal amplitude region and fans out towards the small tidal amplitude region. These new findings are of great importance to extend our understandings of the groundwater hydrodynamics in unconfined coastal aquifers.

Effects of land use on groundwater recharge of a loess terrace under long-term irrigation

Understanding the relation between land use and groundwater recharge is of direct interest in areas that are prone to geohazards. This study was performed to characterize the effects of land use on groundwater table (GWT) distribution in the Heifangtai (HFT) loess terrace under long-term flood irrigation, as irrigation intensity was governed by the crop types. In light of the regional temperature-vegetable dryness index (TVDI) and optical images obtained in different seasons over four years, the dataset incorporated the growth cycles of the local crops, which in turn improved the workload and accuracy of land use detection. Irrigation intensities for different crops were recorded during field investigations. A total of 26 electrical resistivity tomography (ERT) profiles were conducted to estimate the GWT distribution of HFT terrace, which was further utilized to reveal the relation between land use (i.e. irrigation) and GWT fluctuations. The results indicated that high local GWT was associated with vegetable fields which had the highest irrigation intensity, and by contrast, low local GWT was attributed to the perennial fruit tree fields with the lowest irrigation intensity. The discharge was monitored intermittently for over three years to analyze the effect on the local GWT close to margin of HFT. The result showed a strong correlation between spring discharge and GWT fluctuations along the margin of HFT, indicating that groundwater recharge was affected by both irrigation and spring discharges. These results suggested that incorporating growth cycles of crops can effectively facilitate the interpretation of remote sensing data for land use detection, and the results can provide useful guidance for improving irrigation programs and for alleviating geohazards in regions that are under long-term irrigation.

The delayed effects of flooding a residential building – case study

The work describes the case of a single-family residential building, which suffered a breakdown due to flooding. Flooded water in the nearby watercourse, did not reach the building directly, but in its vicinity, but raised to a very high level the local groundwater table, which reached locally, even above ground level. This condition was maintained over a longer period of time, causing the soil moisture to rise, and thus its geotechnical parameters deteriorated. The consequence of this, the building underwent an uncontrolled and uneven settlement, which was accompanied by the formation of cracks on the walls. The effects of failure were presented and an attempt was made to identify possible causes of failure. Own study of the substrate was carried out in "in situ" conditions and verification of laboratory tests, and their results are presented on relevant charts and tables.

Hydrological response of loess slopes with reference to widespread landslide events in the Heifangtai terrace, NW China

In the Heifangtai terrace, NW China, flooding irrigation is common for the cropland production and has increased the groundwater table by 20–30 m over the past five decades. It is believed that excessive irrigation has induced a large number of loess landslides in this area. However, these loess landslides responses to hydrological changes have still not been well understood. Therefore, 43 boreholes and 51 2D Electrical resistivity tomography (ERT) profiles with a total length of 40 km have been investigated to analyze the hydrological characteristics for the whole terrace and characterize the effects of hydrological changes on the occurrence of different types of loess landslides. Our main findings are as follows: (1) Three groundwater domes are found in the center and eastern part of the Heitai terrace and the groundwater table is gradually rising in the whole terrace based on the ERT results. (2) The rising rate of groundwater table at the center of the terrace is twice greater than that at the margin of the terrace and it takes approximately 4 months for irrigation water to penetrate from the top surface to the saturated layer. (3) Landslide deposits could block drainage channels of groundwater, increase the localized hydraulic gradient and result in rising of local groundwater table at the edge of the terrace. (4) The resultant hydraulic gradient impels the groundwater from the center to seep out on the eastern and southern margins and new landslides are prone to occur in the main scarp of landslides with rising groundwater table. The type and magnitude of landslides are affected by groundwater seepage and movement. In order to fundamentally control the occurrence of landslides, effective drainage system and the restrained amount of irrigation water are needed.

Sand-grain micromorphology used as a sediment-source indicator for Kharga Depression dunes (Western Desert, S Egypt)

Abstract Roundness and surface-feature characteristics of sand grains collected from two dune ridges in Kharga Depression (southern Egypt) were tested for potential use as source-to-sink indicators of dunes movement. Grain examination was accommodated through Scanning Electron Microscope (SEM) analysis. Five grain types were distinguished: A) fresh; B) sheet precipitated with “raindrop” structures; C) platy precipitated; D) broken; and E) with chemically etched surfaces—each type diagnostic of a specific geomorphic inheritance. Regarding the level of sphericity, these grains were subdivided into nine roundness classes (0.1–0.9), where angular grains are marked by 0.1 and very well-rounded grains by 0.9. Significant roundness and grain-type surface variations are observed both along dune ridges and between them. Poorly and medium-rounded grain populations dominate, along with sheet-precipitated grains. The contribution of well- and very well-rounded grains is low. The northern part of both eastern and western dune ridges is characterized by grains that represent high-energy aqueous environments with well-rounded grains, whereas platy precipitated grains with a lower level of roundness are concentrated in the middle part of the dune ridges. The southern part of the Kharga Depression is again characterized by sheet-precipitated grains. Our results indicate that the northern part of dune ridges in the Kharga Depression is mainly built of sands that originate from beyond the depression ( e.g., Ghard Abu-Maharik) and the weathered deposits of the Nubian and Moghra Sandstones. The dunes in central and southern part of the Kharga Depression also derive sand from a local depression bottom comprised of playa and fluvial deposits. The growing importance of the local sand source may be explained by the lowering of the local groundwater table, which resulted in playa drying. This groundwater loss resulted in the degradation of the vegetation cover, facilitating an increase in wind entrainment of playa sediments.

Mission Impossible? Maintaining regional grain production level and recovering local groundwater table by cropping system adaptation across the North China Plain

Insufficient precipitation and continuous over-exploitation of groundwater for agricultural irrigation led to rapid drop of groundwater table in a large part of the North China Plain (NCP), the bread basket of China. It has become widely acknowledged that current practice of winter wheat-summer maize sequential cropping system (WM-S) in the NCP will have to come to an end as soon as possible. Great research efforts have been made at the local level via both field experiments and model simulations to construct groundwater neutral cropping systems but virtually all such constructs show a substantial penalty on total output per unit of land per year. In this research, we propose a strategy to meet the double challenge of maintaining regional grain production level and recovering local groundwater table: 1) Widely adopt winter fallow and early-sowing summer maize monocropping (E-M) in water scarce part of the region to enable groundwater recovery; 2) replace WM-S by wheat-maize relay intercropping system (WM-R) in the water richer part of the NCP to increase grain production so as to compensate yield losses in the water scarce part of the region. Our simulations using DSSAT 4.6 at the site level show that both yield and water productivity of E-M are 33.7% and 41.8% higher than those of existing summer maize, with less than 20% of increase in water requirement. In comparison with spring maize, E-M requires 62.4% less irrigation water, with a yield penalty of only 4.52%. At the regional scale, the simulations targeting at maximizing groundwater saving in water scarce area subject to maintaining the current level of regional total output indicate that about 20.45% of the wheat planting area can be put on fallow in winter, most of which is located in the driest regions of the NCP. This can result in a large amount of groundwater saving at 5.62×109m3 and a substitution of wheat by maize at 24.3% of the total wheat output. These findings provide new rooms for the relevant policy makers and stakeholders to address the urgent groundwater recovering issues in the northern NCP without compromising the level of food grain production of the region.

Pleistocene Deposits in the Southern Egyptian Sahara: Lithostratigraphic Relationships of Sediments and Landscape Dynamics at Bir Tarfawi

Abstract The sedimentological and lithostratigraphic record from north-central Bir Tarfawi documents the presence of Pleistocene basin-fill deposits. Three topographic basins were created as a result of deflation during climate episodes associated with lowering of the local groundwater table. In each case, the three deflational basins or topographic depressions were subsequently filled with sediments; these basin aggradations coincided with changes from arid climate conditions to wetter conditions and a rise in the groundwater table. The oldest and highest sedimentary remnant is associated with Acheulian artifacts and may reflect spring-fed pond and marsh conditions during a Middle Pleistocene wet climate episode. Lithofacies for a lower stratigraphic sequence (the “White Lake”) documents deposition in a perennial lake that varied in extent and depth and is associated with Middle Paleolithic artifacts. A third episode of deflation created a topographic low that has been filled with Late Pleistocene sediments that are associated with Middle Paleolithic artifacts and fossil remains. Lateral and vertical variations in the lithofacies of this basin-fill sequence and the sediments of the “grey-green” lake phases provide a record of changing hydrologic conditions. These hydrologic conditions appear to reflect variations in water-table levels related to groundwater recharge and, at times, local rains.

An enhanced SWAT wetland module to quantify hydraulic interactions between riparian depressional wetlands, rivers and aquifers

This study develops a modified version of the Soil and Water Assessment Tool (SWAT) designed to better represent riparian depressional wetlands (SWATrw). It replaces existing unidirectional hydrological interactions between a wetland and a river/aquifer with a more robust bidirectional approach based on hydraulic principles. SWATrw incorporates a more flexible wetland morphometric formula and a connecting channel concept to model wetland-river interactions. SWAT and SWATrw were tested for the Barak-Kushiyara River Basin (Bangladesh and India). Although the two models showed small differences in simulated stream flow, SWATrw outperformed SWAT in reproducing river stages and the pre-monsoon river-spills into riparian wetlands. SWATrw showed that the observed presence of dry season water in the wetland was due to reduced seepage to the local groundwater table whilst continuous seepage simulated by SWAT resulted in the wetland drying out completely. The new model therefore more closely simulates the hydrological interactions between wetlands, rivers and groundwater.

Could alluvial knickpoint retreat rather than fire drive the loss of alluvial wet monsoon forest, tropical northern Australia?

AbstractDrainage rejuvenation through headward migration of alluvial knickpoints is common in ephemeral semi‐arid streams, but has not yet been described for tropical rivers. In the Australian monsoon tropics (AMT), wet monsoon forests have an important ecological function, and are present along many alluvial valleys and springs within a eucalypt‐savanna dominated landscape. Using a combination of LiDAR, remote sensing and field evidence, we observe the ongoing destruction of wet monsoon forest through hydro‐geomorphic feedbacks, along with the headward retreat of an alluvial knickpoint at Wangi Creek in Litchfield National Park, Northern Territory. Due to the highly transmissive shallow aquifer along the lower Wangi Creek, this knickpoint retreat leads to a downstream drop in in‐channel water level, which in turn drives a decrease in the local groundwater table. The lowered groundwater level causes the shallow anabranches and formerly water saturated peaty floodplain soil to desiccate, which results in a reduction of vegetation density. The resulting dry surface conditions allow annual to bi‐annual high frequency low‐intensity fires to affect the monsoon forest, while wet rainforest upstream of the knickpoint remains intact. In this paper, we argue that such hydro‐geomorphic feedbacks may cause the initial destabilization of the forest, which then provides the necessary conditions for the impact of fire. This scenario thus challenges the prevalent view that fire is a first‐order control on the spatial extent of wet monsoonal rainforest in the study area, and provides a new and testable hypothesis for further studies in the AMT. Copyright © 2016 John Wiley & Sons, Ltd.

Coupling of a distributed hydrological model with an urban storm water model for impact analysis of forced infiltration

Summary Only few studies have attempted to couple a storm water runoff model with a distributed hydrological model even though infiltration or exfiltration processes between pipes and canals of urban runoff systems and groundwater are widely recognised. We present a fully coupled model that allows simulation of the complete urban freshwater cycle including: runoff from paved and impervious areas, flow through the runoff network, overland flow, infiltration through the unsaturated zone, evapotranspiration (at green areas), and groundwater flow in complex, urban geology. For example, at the investigated urban area at the City of Silkeborg, Western Denmark, the coupled model show that one fourth (24%) of water input to the storm water runoff systems arrives from groundwater sources. The study furthermore quantifies groundwater feedback mechanisms of forced infiltration to surface water systems by the fully coupled hydrological and urban runoff model. Three local area recharge scenarios with forced infiltration are compared with the present situation without forced infiltration. The forced infiltration impacts the local groundwater table with an average rise of up to 69 cm resulting in significant feedback from the groundwater to the runoff system via drains, overland flow and leakage of groundwater to the pipes and canals of the urban runoff network.

Landscape potential for the adoption of crop cultivation: Role of local soil properties and groundwater table rise during 6000–5400 BP in Flevoland (central Netherlands)

This paper presents a new perspective for the temporal variation of crop cultivation adoption during the period 6000–5400 BP in Flevoland (central Netherlands) based on the spatial evolution of soils and hydrology. To determine the role of soil properties and groundwater level rise in the adoption of crop cultivation in wetlands, mapping of the mid-Holocene palaeotopography, palaeohydrology, soil conditions and distribution of Swifterbant archaeological remains was combined with grain size analyses of cored sediments in two selected study areas. Results show that the low natural fertility and water holding capacity of the sandy soils on the Pleistocene surface in southern Flevoland compared to the loamy soils in the northern part of Flevoland, can explain the lack of crop cultivation in the south. The relation between natural soil fertility and initial adoption of crop cultivation in the IJssel/Vecht valley corresponds with examples from other wetlands in northwest Europe.

Magnitude, geomorphologic response and climate links of lake level oscillations at Laguna Potrok Aike, Patagonian steppe (Argentina)

Laguna Potrok Aike is a large maar lake located in the semiarid steppe of southern Patagonia known for its Lateglacial and Holocene lake level fluctuations. Based on sedimentary, seismic and geomorphological evidences, the lake level curve is updated and extended into the Last Glacial period and the geomorphological development of the lake basin and its catchment area is interpreted.Abrasion and lake level oscillations since at least ∼50 ka caused concentric erosion of the surrounding soft rocks of the Miocene Santa Cruz Formation and expanded the basin diameter by approximately 1 km. A high lake level and overflow conditions of the lake were dated by luminescence methods and tephra correlation to the early Lateglacial as well as to ∼45 ka. The lowest lake level of record occurred during the mid-Holocene. A further lake level drop was probably prevented by groundwater supply. This low lake level eroded a distinct terrace into lacustrine sediments. Collapse of these terraces probably caused mass movement deposits in the profundal zone of the lake. After the mid-Holocene lake level low stand a general and successive transgression occurred until the Little Ice Age maximum; i.e. ca 40 m above the local groundwater table. Frequent lake level oscillations caused deflation of emerged terraces only along the eastern shoreline due to prevailing westerly winds. Preservation of eolian deposits might be linked to relatively moist climate conditions during the past 2.5 ka.Precisely dated lake level reconstructions in the rain-shadow of the Andes document high Last Glacial and low Holocene lake levels that could suggest increased precipitation during the Last Glacial period. As permafrost in semiarid Patagonia is documented and dated to the Last Glacial period we argue that the frozen ground might have increased surficial runoff from the catchment and thus influenced the water balance of the lake. This is important for investigating the glacial to Holocene latitudinal shift and/or strengthening of the Southern Hemispheric Westerlies by using lake level reconstructions as a means to assess the regional water balance. Our interpretation explains the contradiction with investigations based on pollen data indicating drier climatic conditions for the Last Glacial period.

Hydrological impacts of rainwater harvesting (RWH) in a case study catchment: The Arvari River, Rajasthan, India. Part 1: Field-scale impacts

Rainwater harvesting (RWH), the small-scale collection and storage of runoff to augment groundwater stores, has been seen as a solution to the deepening groundwater crisis in India. However, hydrological impacts of RWH in India are not well understood, particularly at the larger catchment-scale. A key element to grasping RWH impact involves understanding the generated recharge variability in time and space, which is the result of variability in rainfall-runoff and efficiency of RWH structures. Yet there are very few reported empirical studies of the impact of RWH. Catchment-scale impacts are best studied using a water balance model, which would require a basic level of field data and understanding of the variability. This study reports the results of a 2-year field study in the 476 km2 semi-arid Arvari River catchment, where over 366 RWH structures have been built since 1985. Difficulties associated with working in semi-arid regions include data scarcity. Potential recharge estimates from seven RWH storages, across three different types and in six landscape positions, were calculated using the water balance method. These estimates were compared with recharge estimates from monitored water levels in 29 dug wells using the water table fluctuation method. The average daily potential recharge from RWH structures varied between 12 and 52 mm/day, while estimated actual recharge reaching the groundwater ranged from 3 to 7 mm/day. The large difference between recharge estimates could be explained through soil storage, local groundwater mounding beneath structures and a large lateral transmissivity in the aquifer. Overall, approximately 7% of rainfall is recharged by RWH in the catchment, which was similar in the comparatively wet and dry years of the field analysis. There were key differences between RWH structures, due to engineering design and location. These results indicate that recharge from RWH affects the local groundwater table, but also has potential to move laterally and impact surrounding areas. However, the greatest weakness in such analysis is the lack of information available on aquifer characteristics, in addition to geology and soil type.

Hydrochemistry of urban groundwater, Seoul, Korea: The impact of subway tunnels on groundwater quality

Hydrogeologic and hydrochemical data for subway tunnel seepage waters in Seoul (Republic of Korea) were examined to understand the effect of underground tunnels on the degradation of urban groundwater. A very large quantity of groundwater (up to 63 million m 3 year − 1 ) is discharged into subway tunnels with a total length of 287 km, resulting in a significant drop of the local groundwater table and the abandonment of groundwater wells. For the tunnel seepage water samples ( n = 72) collected from 43 subway stations, at least one parameter among pathogenic microbes (total coliform, heterotrophic bacteria), dissolved Mn and Fe, NH 4 +, NO 3 −, turbidity, and color exceeded the Korean Drinking Water Standards. Locally, tunnel seepage water was enriched in dissolved Mn (avg. 0.70 mg L − 1 , max. 5.58 mg L − 1 ), in addition to dissolved Fe, NH 4 +, and pathogenic microbes, likely due to significant inflow of sewage water from broken or leaking sewer pipes. Geochemical modeling of redox reactions was conducted to simulate the characteristic hydrochemistry of subway tunnel seepage. The results show that variations in the reducing conditions occur in urban groundwater, dependent upon the amount of organic matter-rich municipal sewage contaminating the aquifer. The organic matter facilitates the reduction and dissolution of Mn- and Fe-bearing solids in aquifers and/or tunnel construction materials, resulting in the successive increase of dissolved Mn and Fe. The present study clearly demonstrates that locally significant deterioration of urban groundwater is caused by a series of interlinked hydrogeologic and hydrochemical changes induced by underground tunnels.

An assessment of twentieth century tree-cover changes on a southern Swedish peatland combining dendrochronoloy and aerial photograph analysis

In the twentieth century, there have been increases in Scots pine (Pinus sylvestris) coverage on peatlands in southern Sweden. In order to assess possible links to contemporary climate change, aerial photograph analysis and dendrochronology were combined to study recent pine cover changes at Anebymossen, a raised peat bog in south central Sweden. Multi-temporal analyses of three sets of panchromatic images from 1950 to 1993 showed a 30% increase of the marginal pine forest of the raised bog during that time, where a significant increase had occurred in the eastern part. Dendrochronological analyses were performed in two parts: the eastern part and the western part where the increase in pine coverage was considerably lower. The tree-ring growth patterns disclosed that a major event, most likely drainage, had occurred at Anebymossen in 1927, affecting tree growth at both sites. The effect of the drainage, which lowers the local ground-water table and hence promotes tree growth, was growth surges with a doubling of the annual increment rates. The effect was short lived in the western part, but continuous drainage in the eastern part caused growth enhancement until the beginning of the 1980s. We suggest that rather than being caused by climate change, the increase in pine coverage on Anebymossen was initiated by the considerable drainage in 1927. Since most peatlands in southern Sweden have been affected by anthropogenic activities, we suggest that it is difficult to make climatological interpretations of tree cover changes on peatlands using only aerial photograph analyses.

Design of Infiltrating Basin by Soil Storage and Conveyance Capacities

Design of storm water storage basins must take both surface infiltrating flow and subsurface seepage flow into consideration. Standing water can result from the condition that the subsurface seepage capacity is less than the surface infiltrating capacity or the basin storage volume exceeds the soil pore volume underneath the basin. To avoid a prolonged draining operation, an infiltrating basin must be designed under the constraints of the soil pore storage capacity before saturation and the soil conveyance capacity after saturation. In this study, the potential flow model is developed to estimate the soil pore storage volume underneath the basin and to predict the conveyance capacity through the saturated soil medium between the basin and the local groundwater table. The potential flow model for infiltrating water flow provides a quantifiable basis to define the soil constraints and to compare various alternatives at the basin site.

Ground penetrating radar study for hydrogeological conditions related with mining activity

In the Bicholime, Goa, there is an iron-ore mine surrounded by villages and small towns. Exploitation of such an enormous amount of iron ore may disturb the local groundwater table, thereby causing groundwater depletion. However, these effects are dependent mainly on the geological formations in the region and the mining method. The effects of Bicholime mining on the surrounding groundwater resources and subsurface strata conditions are the main objective of this study. For this purpose, ground penetrating radar surveys were carried out to study the possible effects of mining on the hydrological regime of this open-pit mine, and the subsurface conditions in the Bicholime opencast iron-ore mine, Goa. The results of these surveys show that laterite layers exist from the surface to depths varying from 5 to 22 m at all study locations in the mining area, and in most places clay layers exist, except for the GPR section along the profiles DD1 and EE1 in the village at depths of 8–22 m. Due to the presence of impervious clay layers, mining and dewatering in the leasehold areas will most likely not affect the surrounding hydrological regime of this mining area. It is concluded that no hydrological connection exists between the open pits and the surrounding villages, so that sufficient groundwater is available in the village area, in spite of the enormous extraction of iron ore from the underground.

Anomalous chloride flux discharges from Yellowstone National Park

Abstract The chloride concentration of some thermal springs in and adjacent to Yellowstone National Park is constant through time although their discharge varies seasonally. As a result the chloride flux from these springs increases during periods of increased discharge. We believe that this is caused by changes in the height of the local groundwater table, which affects the discharge of the springs but not their chloride concentration. The discharge from Mammoth Hot Springs varies seasonally, but its chloride concentration remains constant. We take this as evidence that this major thermal feature is derived from orifices that are tapping the local water table close to its surface. Three of the four major rivers (Yellowstone, Snake and Falls) exiting the Park also show an increased chloride flux during the spring runoff that cannot be explained solely by the contribution of snowmelt, nor by release of hot-spring-derived chloride stored in the soil during the winter and released in the spring. The increased chloride flux in these rivers is attributed to their draining shallow hot springs similar to those mentioned above. In contrast to the Yellowstone, Snake and Falls Rivers, the Firehole and Gibbon Rivers, which unite to form the Madison River and which collectively drain several major geyser basins, display a poor correlation between chloride flux and discharge. The cause, we believe, is that a large part of the thermal water input to these two rivers originated at great depths where the seasonal variation in the height of the water table had a negligible effect on hot spring discharge. Monitoring of seasonal discharge and chloride concentration of thermal features yields information on the depths at which these thermal features tap the local water table.

DESSICATION AND DESTRUCTION: THE IMMEDIATE EFFECTS OF DE‐WATERING AT ETTON, CAMBRIDGESHIRE

SummaryThis paper presents the first results of a long‐term project to monitor changes in ground water conditions in parts of the East Anglian Fenland. So far the project has been concerned with two sites immediately north of Peterborough, at Borough Fen and Etton. The Borough Fen data are still being assessed, but Etton has provided results which warrant prompt publication.The water levels were monitored in a series of boreholes across the Etton Neolithic enclosure for a period of two years (1982–4), to study the effects of the variation in the local groundwater table on the waterlogged contents of the enclosure ditch.The first part of the paper (CF) assesses the results of this programme. The ambient water table remained at or above the level of the primary and lower secondary deposits in the ditch, thus accounting for the excellent preservation of botanical remains. Within one month of the adjacent quarry pumps being turned on the water table had fallen by up to one metre below the lowest archaeological deposits, and has not recovered since, to the almost immediate detriment of the botanical record.The second part of the paper (MT) discusses the observed variation in the preservation of wood in particular, and describes the effects of de‐watering on the deposits of wood in the enclosure ditch.