Riparian Groundwater Research Articles

How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

AbstractRiparian zones are important buffer zones for streams as they are hotspots of nitrate transformation and removal in agricultural catchments. However, mixing of water from different sources and various transformation processes can complicate the quantification of nitrate turnover in riparian zones. In this study, we analyzed nitrate concentration and isotope data in riparian groundwater along a 2‐km stream section in central Germany. We developed a mathematical model combining end‐member mixing and isotope modeling to account for mixing of river water and groundwater and quantify nitrate transformation in riparian groundwater. This enabled us to explicitly determine the extent of denitrification (as process leading to permanent nitrate removal from riparian groundwater) and transient nitrate removal by additional processes associated with negligible isotope fractionation (e.g., plant uptake and microbial assimilation) and to perform an extensive uncertainty analysis. Based on the nitrogen isotope data of nitrate, the simulations suggest a mean removal of up to 28% by additional processes and only about 9% by denitrification. Nitrate removal from riparian groundwater by additional processes exceeded denitrification particularly in winter and at larger distance from the river, underlining the role of the river as organic carbon source. This highlights that nitrate consumption by additional processes predominates at the field site, implying that a substantial fraction of agricultural nitrogen input is not permanently removed but rather retained in the riparian zone. Overall, our model represents a useful tool to better compare nitrogen retention to permanent nitrogen removal in riparian zones at various temporal and spatial scales.

Seasonal evolution of active layer thaw depth and hillslope‐stream connectivity in a permafrost watershed

AbstractTo advance our understanding of permafrost hillslope drainage dynamics and its influence on streamflow hydrochemistry, we instrumented a hillslope‐stream sequence located in the headwaters of the Niaqunguk River watershed, Nunavut, Canada (63°N, 68°W). We combined high spatial resolution field measurements of water and frost tables across the hillslope, with semiweekly measurements of groundwater and streamflow chemistry to track the evolution of streamflow chemistry during active layer thaw. Interestingly, localized differential thaw patterns emerged under near saturation conditions across the instrumented hillslope, the result of a low‐frequency high‐magnitude summer rainfall event. Hillslope structure and uneven active layer thaw created two distinct fill‐and‐spill domains. A subsurface domain defined by frost table microtopography and a surface domain defined by surface topography. We observed a seasonal shift in streamflow chemistry with an increased influence of water flowing through the underlying mineral soils as the active layer thawed. As thaw progressed streamflow chemistry began to match that of the riparian groundwater, which was a mixture of hillslope surface and subsurface water. Hillslope‐stream surface connections were sporadic and occurred when rainfall and saturation conditions across the lower portion of the hillslope were sufficient for water to spill out of midslope surface depressions and across a saturated riparian zone and into the stream. This research shows how hillslope structure and thaw processes influence hillslope‐stream connectivity in permafrost terrain.

Groundwater-surface water interaction along river Kali, near Aligarh, India

River Kali, in the western gangetic plains, is posing serious threat to riparian communities by contaminating shallow riparian zone groundwater. To manage groundwater contamination, this study attempts to investigate the nature of groundwater-river water interaction, during February to April 2018, along a segment of river Kali near Aligarh in western Uttar Pradesh, India. River discharge was measured at three locations in two phases. Hydraulic heads in mini-piezometers were observed twice weekly. Forty water samples were analysed for major ions, nitrate, fluoride, TDS, pH, EC. River discharges and river bank vertical hydraulic gradients reveal that river Kali is losing water to riparian aquifers. Hydro-chemical analysis shows discordance in the pattern of ionic concentrations of river and groundwater, a distinct clustering of river and groundwater for both sampling periods. Ionic concentrations in groundwater with distance from channel indicate potentially clogged riverbed conditions that reduce the interaction of river water with riparian groundwater.

Macroinvertebrates and Microbes (Archaea, Bacteria) Offer Complementary Insights into Mine-Pit Lake Ecology

The broad objective of this research was to determine the environmental drivers of macroinvertebrate and microbial assemblages in acidic pit lakes. This is important because pit lake ecosystem development is influenced by prevailing environmental characteristics. Three lakes (Stockton, Kepwari, WO5H) within a larger pit-lake district in Collie, Western Australia were surveyed for spatial variability of benthic macroinvertebrate and microbe (Archaea, Bacteria) assemblage composition as well as potential environmental drivers (riparian condition, aquatic habitat, sediments, and aquatic chemistry) of assemblages. With the exception of sediment chemistry, biophysical variables were significantly different across lakes and reflected riparian condition and groundwater chemistry. Microbial assemblages in pit lakes were significantly different across lakes and correlated with water chemistry, particularly metals in Lake WO5H. However, the most abundant microbes were not readily identified beyond class, making it difficult to speculate on their ecological function. Macroinvertebrate assemblage composition and species richness were also significantly different across all lakes, and in Lake WO5H (a lake with low pH and high metal concentrations), taxa were correlated with benthic organic matter as well as water chemistry. Results indicated that despite poor water quality, input of nutrients from terrestrial leaf litter can support or augment pit lake ecosystems. This is a demonstration of the concept that connection of pit lakes to catchments can positively affect aquatic ecosystems, which can inform management actions for remediation.

Spectral Decomposition Reveals New Perspectives on CO2Concentration Patterns and Soil‐Stream Linkages

AbstractThe rapid development of novel technologies to obtain high‐frequency observations has provided new possibilities to observe and understand carbon cycling in inland waters. This study investigates carbon dioxide (CO2) dynamics along a boreal soil‐stream transect using a state‐of‐the‐art data set in combination with a spectral methodology to identify controls on stream CO2. The spectral decomposition of hourly observations revealed intermediate (multiday) to long‐term (monthly) patterns across the upslope‐riparian‐stream continuum, with similar power law increases in CO2concentration fluctuations with increasing period. High‐frequency CO2variabilities, specifically diel CO2concentration fluctuations, were also identified at all locations but were substantially amplified in the stream compared to in the riparian groundwater. Moreover, the spectral coherence between soil and stream CO2fluctuations was inconsistent and restricted to episodic events. In contrast, we found a strong and consistent spectral coherence between the riparian groundwater level and stream CO2concentration, indicating a hydrological control on stream CO2dynamics. However, during some time spans even these patterns were obscured, suggesting that additional processes, such as CO2evasion and in‐stream metabolism, modulated the influence from riparian sources. The scales and patterns of temporal coherences (or lack thereof) between CO2at different points in the catchment, as well as with other factors, for example, groundwater levels and Photosynthetically Active Radiation, provide new perspectives on the range of processes governing stream CO2dynamics. Thus, this study highlights the potential of using spectral decomposition of high‐resolution, spatially distributed data of different types to investigate biogeochemical transformations and pathways linking terrestrial and aquatic systems.

Water transit time and active recharge in the Sahel inferred by bomb-produced 36Cl

The lack of data and suitable methods to quantify regional hydrological processes often hinders sustainable water management and adaptation to climate change in semiarid regions, particularly in the Sahel, which is known for its climatic variability. Here we show that 36Cl from nuclear tests is a promising method to estimate water transit times and groundwater recharge rates on the catchment scale, and to distinguish water and chloride cycles. 36Cl was measured in 131 surface and groundwater samples in the Chari-Logone sub-catchment of the emblematic Lake Chad Basin, located in central Sahel. It was found that only 12 ± 8% of the catchment is connected to the main rivers. Groundwater supporting rivers in the upper humid part of the catchment has a mean transit time of 9.5 ± 1 years and a recharge rate of 240 ± 170 mm yr−1. In the lower Sahelian part of the catchment, stream-focused recharge yields recharge rates up to 78 ± 7 mm yr−1 in riparian groundwater against 16 ± 27 mm yr−1 elsewhere. Our estimates suggest that aquifers in the Sahel host a significant amount of renewable water, which could therefore be used as a strategic freshwater resource.

River recharge versus O2 supply from the unsaturated zone in shallow riparian groundwater: A case study from the Selke River (Germany)

Besides gas-water-exchange in surface waters, respiratory consumption of dissolved oxygen (DO) in adjacent riparian groundwater may trigger the addition of so far hardly explored sources from the unsaturated zone. These processes also systematically influence stable isotope ratios of DO and were investigated together with Cl− as a conservative tracer for water mixing in a near-river riparian groundwater system. The study focused on a losing stream section of the Selke River at the foot of the Harz Mountains (Germany). The study area exposed steep DO gradients between the stream water and riparian groundwater between April 2016 and May 2017. Our results indicated dominant influences of microbial community respiration with observed DO concentration gradients. These observations can be explained by DO from the river that is subject to fractionation by microbial respiration with a typical fractionation factor (αr) of 0.982. However, with such respiration dominance, we expected a simultaneous enrichment of δ18ODO towards values that are more positive than the well-known atmospheric O2 signal of +23.9‰ versus the Vienna Standard Mean Ocean Water standard (VSMOW). Surprisingly, our measurements revealed much lower δ18ODO values between +22‰ and +18‰ in the near-river groundwater. Mass balance calculations revealed that the DO pool in the shallow and unconfined aquifer receives contributions of up to about 80% by diffusion of oxygen from the vadose zone with a distinctly lower isotope value than the one of the atmosphere. This finding about additional oxygen sources from the unsaturated zone has numerous ramifications for oxygen related processes in near-river environments including the oxidation of pollutants, nutrients and ecosystem health.

The effects of floods on the temperature of riparian groundwater

AbstractThe transition area between rivers and their adjacent riparian aquifers, which may comprise the hyporheic zone, hosts important biochemical reactions, which control water quality. The rates of these reactions and metabolic processes are temperature dependent. Yet the thermal dynamics of riparian aquifers, especially during flooding and dynamic groundwater flow conditions, has seldom been studied. Thus, we investigated heat transport in riparian aquifers during 3 flood events of different magnitudes at 2 sites along the same river. River and riparian aquifer temperature and water‐level data along the Lower Colorado River in Central Texas, USA, were monitored across 2‐dimensional vertical sections perpendicular to the bank. At the downstream site, preflood temperature penetration distance into the bank suggested that advective heat transport from lateral hyporheic exchange of river water into the riparian aquifer was occurring during relatively steady low‐flow river conditions. Although a small (20‐cm stage increase) dam‐controlled flood pulse had no observable influence on groundwater temperature, larger floods (40‐cm and >3‐m stage increases) caused lateral movement of distinct heat plumes away from the river during flood stage, which then retreated back towards the river after flood recession. These plumes result from advective heat transport caused by flood waters being forced into the riparian aquifer. These flood‐induced temperature responses were controlled by the size of the flood, river water temperature during the flood, and local factors at the study sites, such as topography and local ambient water table configuration. For the intermediate and large floods, the thermal disturbance in the riparian aquifer lasted days after flood waters receded. Large floods therefore have impacts on the temperature regime of riparian aquifers lasting long beyond the flood's timescale. These persistent thermal disturbances may have a significant impact on biochemical reaction rates, nutrient cycling, and ecological niches in the river corridor.

Decoupling of dissolved organic matter patterns between stream and riparian groundwater in a headwater forested catchment

Abstract. Streams are important sources of carbon to the atmosphere, though knowing whether they merely outgas terrestrially derived carbon dioxide or mineralize terrestrial inputs of dissolved organic matter (DOM) is still a big challenge in ecology. The objective of this study was to investigate the influence of riparian groundwater (GW) and in-stream processes on the temporal pattern of stream DOM concentrations and quality in a forested headwater stream, and whether this influence differed between the leaf litter fall (LLF) period and the remaining part of the year (non-LLF). The spectroscopic indexes (fluorescence index, biological index, humification index, and parallel factor analysis components) indicated that DOM had an eminently protein-like character and was most likely originated from microbial sources and recent biological activity in both stream water and riparian GW. However, paired samples of stream water and riparian GW showed that dissolved organic carbon (DOC) and nitrogen (DON) concentrations as well as the spectroscopic character of DOM differed between the two compartments throughout the year. A simple mass balance approach indicated that in-stream processes along the reach contributed to reducing DOC and DON fluxes by 50 and 30 %, respectively. Further, in-stream DOC and DON uptakes were unrelated to each other, suggesting that these two compounds underwent different biogeochemical pathways. During the LLF period, stream DOC and DOC : DON ratios were higher than during the non-LLF period, and spectroscopic indexes suggested a major influence of terrestrial vegetation on stream DOM. Our study highlights that stream DOM is not merely a reflection of riparian GW entering the stream and that headwater streams have the capacity to internally produce, transform, and consume DOM.

Using environmental tracers and modelling to identify natural and gas well-induced emissions of methane into streams

Development of shale gas wells sometimes results in migration of methane (CH4) from boreholes into aquifers. Identification of leakage has relied on analysis of CH4 in individual groundwater samples, usually from water wells; however, collection of point data is expensive and prone to artefacts. Methane analysis in streams is a novel way to find potential leakage of CH4 to groundwater. Here, dissolved hydrocarbons and geochemical tracers were measured in streams during base flow in watersheds with high densities of shale gas wells in Pennsylvania (PA) to identify characteristics related to leaking gas wells. Three streams with no known contamination from gas wells and one stream near a gas well previously reported to be leaking CH4 were investigated. The characteristics observed in the stream near the putatively leaking gas well that distinguish it from the streams without leaks include higher CH4 concentrations in riparian groundwater (as high as 4600 versus 206 μg/L), a relatively high gas influx to the stream channel (>70 versus <10 mg m−2 d−1), hydrocarbon isotopic signatures and radiogenic strontium consistent with Middle Devonian Marcellus Formation shale, and higher concentrations of modern atmospheric age tracers in groundwater. These tracer concentrations may indicate upward transport of hydrocarbons as a separate gas phase rather than in solution. In addition, the stream near the putatively leaking well was not located along a fault-related topographic lineament whereas streams with substantial natural thermogenic CH4 influxes tend to be aligned with potential geologic structures. The stream approach is an efficient technique to estimate watershed-scale groundwater compositions and fluxes of CH4 that reveal natural and anthropogenic sources of methane emissions.

Spatial delineation of riparian groundwater within alluvium deposit of mountainous region using Laplace equation

AbstractThe interaction between surface and groundwater plays a key role in a riparian ecosystem while the size of riparian groundwater has not been typically incorporated into hydrological modelling systems. An extensive geophysical survey composed of 25 individual DC electrical resistivity profiles was conducted at the Blair–Wallis site in Wyoming. The observed resistivity images show a near‐surface aquifer interpreted as the saturated alluvium deposit along the channel, rather than the geological bedrock. Based on the electrical resistivity images, it can be inferred that only the near‐surface portion of the groundwater actively interacts with the stream flow in the mountainous and hilly watershed. This study attempted the spatial extrapolation of the measured riparian aquifer depths by means of fitting functions based on the surface topography. The analysis indicated that the boundary of the riparian aquifer well corresponds to the topographical inflexion point of the hill slope profile. It was also demonstrated that the extent of alluvium deposit, where the area of riparian aquifer is indicated, can be delineated using the slope and curvature maps in the geographic information system. Then, the parabolic and biharmonic functions were tested for the groundwater depth estimation using the developed alluvium deposit map. The proposed methodology was effective if geological diffusion processes by wind and water dominated the topography. The spatial map of the active aquifer will be useful in hydrological drought analysis because it is considered to be a main source of baseflow during dry seasons.

River water infiltration enhances denitrification efficiency in riparian groundwater

Nitrate contamination in ground- and surface water is a persistent problem in countries with intense agriculture. The transition zone between rivers and their riparian aquifers, where river water and groundwater interact, may play an important role in mediating nitrate exports, as it can facilitate intensive denitrification, which permanently removes nitrate from the aquatic system. However, the in-situ factors controlling riparian denitrification are not fully understood, as they are often strongly linked and their effects superimpose each other. In this study, we present the evaluation of hydrochemical and isotopic data from a 2-year sampling period of river water and groundwater in the riparian zone along a 3rd order river in Central Germany. Based on bi- and multivariate statistics (Spearman's rank correlation and partial least squares regression) we can show, that highest rates for oxygen consumption and denitrification in the riparian aquifer occur where the fraction of infiltrated river water and at the same time groundwater temperature, are high. River discharge and depth to groundwater are additional explanatory variables for those reaction rates, but of minor importance. Our data and analyses suggest that at locations in the riparian aquifer, which show significant river water infiltration, heterotrophic microbial reactions in the riparian zone may be fueled by bioavailable organic carbon derived from the river water. We conclude that interactions between rivers and riparian groundwater are likely to be a key control of nitrate removal and should be considered as a measure to mitigate high nitrate exports from agricultural catchments.

Mapping riparian vegetation and charactersing its groundwater dependency at the modder river government water scheme

Groundwater use by riparian vegetation transpiration commonly results in a drawdown of the water table which may significantly reduce groundwater discharge to streams and even change the status of streams from gaining to losing. Therefore, the significance of the riparian transpiration process, due to the continuously increasing demand for integrated sustainable water resources management policies, deserves great attention. It is for this reason that the present work was evoked to research the riparian groundwater dependency and consumptive groundwater use by the streamside flora. Observations of the daily groundwater multiple hydrographs, in conjunction with average daily temperature and solar radiation, indicated that the groundwater fluctuations were due to water losses from the riparian flora. The Diospyros lycioides (shrub) and Acacia karroo (tree) communities were found to be the two dominant riparian species along the subject area. However, the results indicated that only the Acacia karroo sourced their water primarily from the groundwater storage because: 1) the isotopic signature of twig xylem water was similar to that of groundwater and 2) the rooting depth was at the water table. This species was found to abstract approximately 121,100m3/month from the riparian groundwater. This is considerably a voluminous abstraction which is most likely to pose a problem of water competition; therefore, groundwater dependency of the riverine flora must be considered in the formulation of any integrated sustainable water resources management policies in the subject area and/or similar areas for the co-existence of human society and nature.

Hydrological connectivity drives dissolved organic matter processing in an intermittent stream

Hydrological conditions are key drivers of dissolved organic matter (DOM) processing in intermittent streams. However, there still exist major gaps in knowledge regarding the temporal dynamics of DOM processing during drought periods, as well as the role of the hyporheic zone (HZ). We conducted weekly sampling of surface water and hyporheic pore water during a drying/rewetting cycle and characterized DOM by fluorescence and absorbance properties. Overall, the contribution of allochthonous and humic-like DOM increased during base flow in early summer (pre-drought) and continued increasing throughout the drought period, which covered three phases: contraction, fragmentation and dry. The contribution of autochthonous DOM during this period was restricted to very specific points in time (the transition from contraction to fragmentation phase) and space (the HZ). Hydrological connectivity between the HZ and the surface water was a driver of DOM composition by supplying terrestrial, aromatic DOM to the HZ. The disconnection of the stream from the riparian groundwater enabled us to quantify the DOM retention/release in the HZ. DOM mass balance at the stream-hyporheic interface revealed the occurrence of two time periods with disproportionately high rates for DOM processing (hot moments) during the study period: (1) a short pulse of protein-like, autochthonous DOM net release at the beginning of the disconnection; and (2) a longer time period of increasing net dissolved organic carbon (DOC) retention up to 30% along 25m of HZ length during fragmentation and dry phase. Remarkably, the net carbon retention was coupled to a decrease of aromatic and high molecular weight compounds, while protein-like, autochthonous DOM was released. This result evidenced that under drought conditions, the HZ becomes a sink for DOM compounds previously assumed to be recalcitrant in aquatic ecosystems and therefore highlights the importance of hydrological drivers on DOM processing.

Ecological effects from groundwater contaminated by volatile organic compounds on an urban stream’s benthic ecosystem

While the discharge of groundwater contaminated with volatile organic compounds to surface waters is widely reported, assessments of its ecological threat are rare in the scientific literature, largely being restricted to comparisons of riparian groundwater concentrations to water quality guidelines for protecting aquatic life. Here we investigated potential negative impacts on the benthic community from overlapping petroleum hydrocarbon – chlorinated solvent plumes discharging to an urban stream using in-field ecological assessment methods (targeting both meio- and macrobenthos) over multiple scales. Several lines of evidence were suggestive of detrimental impacts from the plumes, including reduced total abundance and richness of benthic taxa, reduced or enhanced abundances of individual benthic taxa, and an altered benthic community structure. However, the findings were not conclusive, as the evaluation was complicated by substantial small-scale (<20m) spatial variation in concentrations of many other groundwater contaminants, making it difficult to determine reference areas (i.e., those with similar substrate and chemistry, but lacking exposure to plume compounds) and isolate impacts from a single factor. Furthermore, detections of the wastewater indicator acesulfame also demonstrated the issue of uncertain influences from unmeasured contaminants (e.g., pharmaceuticals). Some of the assessment methods showed greater promise than others (e.g., sampling along a gradient rather than having separate impacted and reference sites), which may provide guidance on future field applications. Overall, the study findings suggest a need for further inquiry into the most appropriate application of ecological and ecotoxicological assessment methods to groundwater-based contamination hazards.

Spatio-temporal streamflow generation in a small, steep headwater catchment in western Japan

ABSTRACTHeadwaters contribute a substantial part of the flow in river networks. However, spatial variations of streamflow generation processes in steep headwaters have not been well studied. In this study, we examined the spatio-temporal variation of streamflow generation processes in a steep 2.98-ha headwater catchment. The time when baseflow of the upstream section exceeded that downstream was coincident with the time when the riparian groundwater switched from downwelling to upwelling. This suggests that upwelling of the riparian groundwater increased considerably in the upstream section during the wet period, producing a shift in the relative size of baseflow between the upstream and downstream sections. The timing of fluctuations among hillslope soil moisture, hillslope groundwater and streamflow reveals that the hillslope contributed to storm flow, but this contribution was limited to the wet period. Overall, these results suggest that streamflow generation has strong spatial variations, even in small, steep headwater catchments.EDITOR A. Castellarin ASSOCIATE EDITOR X. Chen

Long-term effects of stump harvest on mercury and general chemistry in discharge water

ABSTRACTThis synoptic study evaluates water chemistry in stump-harvested and conventionally logged reference areas, located in South-Central Sweden, 20–30 years after harvest and reforestation. Electrical conductivity, pH, alkalinity (), nitrate nitrogen () and total organic carbon (TOC) were measured in discharging groundwaters and in low-order streams and ditches. The groundwaters were further analysed for total mercury (THg) and methyl mercury (MeHg). Additionally, solid phase Hgtot and MeHg were measured in stream mudbottoms. The mean MeHg concentration and mean MeHg/TOC ratio were significantly (p < .05) higher in discharging groundwater from stump-harvested compared to reference areas. In contrast, the concentration of THg did not differ. Likewise, there were no significant treatment differences for electrical conductivity, pH, or TOC. In the stream-/ditchwaters also no significant differences were found, but electrical conductivity (p = 0.07) and pH (p = .08) tended to be lower in stump-harvested areas. Hgtot and MeHg concentrations in stream-/ditch sediments did not differ between treatments. These results suggest that weakly enhanced MeHg concentrations may be present in the riparian groundwater zone 20–30 years after stump harvest.

THE ROLE OF SUBSURFACE FLOW DYNAMIC ON SPATIAL AND TEMPORAL VARIATION OF WATER CHEMISTRY IN A HEADWATER CATCHMENT

Variation of water chemistry does not merely occur due to in situ chemical process, but also transport process. The study was carried out to address the role of subsurface flow dynamic on spatial and temporal variation of water chemistry in a headwater catchment. Hydrometric and hydrochemistry measurements were done in transect with nested piezometers, tensiometers, and suction samplers at different depths across hillslope and riparian zone in a 5.2 ha first-order drainage of the Kawakami experimental basin, Nagano, Central Japan from August 2000 to August 2001. Spatial variation of solute concentration was defined by the standard deviation and coefficient of variation of the seasonal observed concentrations. Autocorrelation analysis was performed to define temporal variation of solute concentration. The results showed that spatial variation of water chemistry was mainly influenced by the variation of subsurface flow through the hillslope and riparian zone. Solute concentration in the deep riparian groundwater was almost three times higher than that in the hillslope segment. A prominent downward flow in deep riparian groundwater zone provided transport of solutes to the deeper layer. Time series analysis showed that in the deep riparian groundwater, Ca2+, Mg2+, SO42- and HCO3- concentrations underwent a random process, Na+ concentration of a random process superimposed by a trend process, and SiO2 of a random process superimposed by a periodic process. Near the riparian surface, SO42- concentration was composed of a random process superimposed by a periodic process, whereas other solutes were mainly in a random process. In the hillslope soil water, there was no trend observed for the Na+ concentration, but there were for Ca2+ and Mg2+. The magnitude and direction of subsurface flow across hillslope and riparian zone created transport and deposition processes that changed solute concentration spatially and temporally.

THE ROLE OF SUBSURFACE FLOW DYNAMIC ON SPATIAL AND TEMPORAL VARIATION OF WATER CHEMISTRY IN A HEADWATER CATCHMENT

Variation of water chemistry does not merely occur due to in situ chemical process, but also transport process. The study was carried out to address the role of subsurface flow dynamic on spatial and temporal variation of water chemistry in a headwater catchment. Hydrometric and hydrochemistry measurements were done in transect with nested piezometers, tensiometers, and suction samplers at different depths across hillslope and riparian zone in a 5.2 ha first-order drainage of the Kawakami experimental basin, Nagano, Central Japan from August 2000 to August 2001. Spatial variation of solute concentration was defined by the standard deviation and coefficient of variation of the seasonal observed concentrations. Autocorrelation analysis was performed to define temporal variation of solute concentration. The results showed that spatial variation of water chemistry was mainly influenced by the variation of subsurface flow through the hillslope and riparian zone. Solute concentration in the deep riparian groundwater was almost three times higher than that in the hillslope segment. A prominent downward flow in deep riparian groundwater zone provided transport of solutes to the deeper layer. Time series analysis showed that in the deep riparian groundwater, Ca2+, Mg2+, SO42- and HCO3- concentrations underwent a random process, Na+ concentration of a random process superimposed by a trend process, and SiO2 of a random process superimposed by a periodic process. Near the riparian surface, SO42- concentration was composed of a random process superimposed by a periodic process, whereas other solutes were mainly in a random process. In the hillslope soil water, there was no trend observed for the Na+ concentration, but there were for Ca2+ and Mg2+. The magnitude and direction of subsurface flow across hillslope and riparian zone created transport and deposition processes that changed solute concentration spatially and temporally.

The influence of riparian evapotranspiration on stream hydrology and nitrogen retention in a subhumid Mediterranean catchment

Abstract. Riparian evapotranspiration (ET) can influence stream hydrology at catchment scale by promoting the net loss of water from the stream towards the riparian zone (i.e., stream hydrological retention). However, the consequences of stream hydrological retention on nitrogen dynamics are not well understood. To fill this gap of knowledge, we investigated changes in riparian ET, stream discharge, and nutrient chemistry in two contiguous reaches (headwater and valley) with contrasted riparian forest size in a small forested Mediterranean catchment. Additionally, riparian groundwater level (hgw) was measured at the valley reach. The temporal pattern of riparian ET was similar between reaches, and was positively correlated with hgw (ρ = 0.60) and negatively correlated with net riparian groundwater inputs (ρ &lt; −0.55). During the vegetative period, stream hydrological retention occurred mostly at the valley reach (59 % of the time), and was accompanied by in-stream nitrate release and ammonium uptake. During the dormant period, when the stream gained water from riparian groundwater, results showed small influences of riparian ET on stream hydrology and nitrogen concentrations. Despite being a small component of annual water budgets (4.5 %), our results highlight that riparian ET drives stream and groundwater hydrology in this Mediterranean catchment and, furthermore, question the potential of the riparian zone as a natural filter of nitrogen loads.