Groundwater Baseflow Research Articles

Enhancing streamflow simulations with gated recurrent units deep learning models in the flood prone region with low-convergence streamflow data

Highly accurate streamflow simulations are essential for water resources and river management. However, there are longstanding challenges for streamflow modeling in the hydrology field. As a result, deep learning methods have been introduced as novel tools and are being used in simulation. This paper proposes a novel approach that utilizes unconventional data preprocessing techniques to improve the accuracy of daily streamflow predictions in flood-prone regions. We applied this approach to a flood-prone area in western Iran and simulated daily streamflow using a deep learning Gated Recurrent Unit (GRU) method under various data selection scenarios. These scenarios focused on identifying the optimal combination of input variables, time steps, and outlier removal techniques. The outlier removal methods investigated in this study include Mahalanobis distance, critical section removal, Z-score, and no removal. The average rainfall of the area, data driven precipitation, Normalized Difference Vegetation Index (NDVI), surface soil moisture, groundwater baseflows, and streamflow in the hydrometric station were evaluated using correlation control method, and inputs with the lowest correlation were removed. Based on the results obtained from the deep learning models produced in the research, it was found that the GRU model, with various modified inputs using Z-score removal, had the best performance. The model had an average of Root Mean Squared Error (RMSE) at 5.24 mm and R2 (Coefficient of Determination) at 0.91 during training, while during validation, it had an RMSE of 7.74 mm and R2 of 0.83. Considering using relatively low convergence data in streamflow simulation in this study, it can be said that the listed scenarios showed an appropriate result in dealing with the data and recognizing the complex pattern of the daily streamflow, and the future studies will show the improvement of the GRU models if higher convergence data will be used.

Hydrogeologic and hydrochemical inputs to emerging wetlands on the shores of the receding Salton Sea, California

AbstractThe Salton Sea has experienced significant recession over the past two decades due to changes in the diversion of Colorado River water to the Salton Trough for agricultural irrigation. As a result, wetlands have emerged in some exposed playa areas along the Salton Sea, primarily in regions with extensive agricultural return flows and agricultural drainage. One notable wetland system, known as the Bombay Beach Wetlands, has formed on the north shore of the Salton Sea, in an area devoid of agriculture. In many other areas with limited or no agriculture, wetlands have failed to develop, leaving exposed playa surfaces as the Salton Sea recedes. These dry playa surfaces pose a significant threat to the health of local residents due to the presence of toxins contained in windblown dust associated with playa deposits. In this study, stable water isotope data, combined with other hydrological information, led to identification of two potential water sources for the Bombay Beach Wetlands. The first possibility proposes that thermal artesian waters alone contribute to the wetlands' water source, while the second hypothesis involves a combination of drainage from Salton Sea bank storage water mixing with the thermal artesian water. The thermal artesian water discharges into drainage channels that flow towards the Bombay Beach Wetlands, initially devoid of possible groundwater baseflow until reaching the wetlands. Studies were subsequently done along the full reach of the drainage channels receiving thermal artesian water. Dissolved solids content, P and N nutrients, arsenic, and stable water isotopes were tested synoptically along the drainage channels. Channel investigations led to the development of a novel model of salinization, which is linked to channel discharge, channel morphometrics, and channel incision.

The effects of heavy rain on the fate of urban and agricultural pollutants in the riverside area around weirs using multi-isotope, microbial data and numerical simulation

The increase in extreme heavy rain due to climate change is a critical factor in the fate of urban and agricultural pollutants in aquatic system. Nutrients, including NO3− and PO43−, are transported with surface and seepage waters into rivers, lakes and aquifers and can eventually lead to algal blooms. δ15N-NO3−, δ18O-NO3−, and δ11B combined with hydrogeochemical and microbial data for groundwater and surface water samples were interpreted to evaluate the fate of nutrients in a riverside area around weirs in Daegu, South Korea. Most of the ions showed similar concentrations in the groundwater samples before and after heavy rain while concentrations of major ions in surface water samples were diluted after heavy rain. However, Si, PO43−, Zn, Ce, La, Pb, Cu and a number of waterborne pathogens increased in surface water after heavy rain. The interpretation of δ11B, δ15N-NO3−, and δ18O-NO3− values using a Bayesian mixing model revealed that sewage and synthetic fertilizers were the main sources of contaminants in the groundwater and surface water samples. δ18O and SiO2 interpreted using the Bayesian mixing model indicated that the groundwater component in the surface water increased from 4.4 % to 17.9 % during the wet season. This is consistent with numerical simulation results indicating that the direct surface runoff and the groundwater baseflow contributions to the river system had also increased 6.4 times during the wet season. The increase in proteobacteria and decrease of actinobacteria in the surface water samples after heavy rain were also consistent with an increase of surface runoff and an increased groundwater component in the surface water. This study suggests that source apportionment based on chemical and multi-isotope data combined with numerical modeling approaches can be useful for identifying main hydrological and geochemical processes in riverside areas around weirs and can inform suggestions of effective methods for water quality management.

Effects of dry and heavy rainfall periods on arsenic species and behaviour in the aquatic environment adjacent a mining area in South Korea

This study investigated the spatiotemporal variation of arsenic (As) distribution, species and its behaviour in the aquatic environment changed by extended dry and heavy rainfall periods in the area adjacent an abandoned gold mine, South Korea. As appears to be transported from the mine wastes to Guryong stream through groundwater baseflow and leachate. The oxidation-reduction potential (ORP) conditions changed spatially and temporally following dry and heavy rainfall periods, and rainfalls caused decrease in the stream pH. As mainly existed as arsenates (AsO 4 3- ) bound with H, Ca, and Fe in water. In groundwater, the lower the pH and the ORP, the higher the proportions of acid species of arsenate (HAsO 4 2- , H 2 AsO 4 - ) and arsenite (H 3 AsO 3 ), resulting in increased As mobility and toxicity, respectively. In stream, the primary influencing factor of As variation is the ORP. Under oxidizing conditions, As in stream could precipitate as amorphous FeAsO 4 ·2H 2 O and FeOOH in the streambed. Then, the ORP decrease could remobilize As by redissolution and desorption of As bound to bed sediments. Thus, streambed sediments acted as a temporary sink-and-source for As, and extension of source areas accompanied with physical transport after heavy rainfalls. • Total As concentration and pH-Eh conditions controlled As behavior in water. • Dryness and rainfalls change the pH and ORP conditions in the aquatic environment. • Streambed sediments acted as a sink-and-source in groundwater-stream interface. • Physical transport of bed sediments could extend the areas of As-pollution potential.

Surface water and groundwater hydraulics of lowland karst aquifers of Estonia

In Estonia, karst aquifers supply roughly a third of annually abstracted domestic groundwater. Karst systems recharged by allogenic humic-rich streams originating from mire-dominated watersheds are common in Lower Estonia (lowlands). This study aimed to characterize the hydrology and hydraulics of two binary lowland karst systems: Salajõe (SKS) and Tuhala karst system (TKS), and their interaction with the hosting karst aquifer. High-resolution water level, discharge, specific conductance time series data combined with tracer test results were analyzed to assess the two systems. The temporal effect of evapotranspiration, snow deposition and snowmelt floods contributed to the variability in recharge event sensitivity of studied watersheds and aquifers, resulting in time-varying CCF strength between precipitation and discharge. The greatest event sensitivity was observed during the autumn and snowless winter seasons. The celerity and solute transport were the greatest in the preferential flow paths of both karst systems, however, solute transport characteristics varied significantly because of differences in structure and maturity of karst porosity. While the dilute allogenic recharge flushed the karst systems occasionally, the contribution of high-SEC groundwater baseflow was significant. During flood events, the matrix-conduit hydraulic gradient could be inverted, resulting in dilute floodwater being pushed into the hosting aquifer from the karst system.

Hydrological Modelling for Water Resource Management in a Semi-Arid Mountainous Region Using the Soil and Water Assessment Tool: A Case Study in Northern Afghanistan

To address the issues of water shortages and the loss of agricultural products at harvest in northern Afghanistan, the Soil and Water Assessment Tool (SWAT) was applied for agricultural water resource management by simulating surface runoff in the Balkhab River basin (BRB) on a monthly basis from 2013 to 2018. Elevation, slope, land cover data, soil maps, and climate data such as temperature, precipitation, relative humidity, wind speed, and solar radiation were used as inputs in the SWAT modelling. During the dry season from July to September, the water resources downstream were basically attributed to baseflow from groundwater. In the calibration, the groundwater baseflow was estimated by analyzing station-recorded discharges for 190 springs. With the estimated baseflow, the SWAT results were markedly improved, with R2 values of 0.70, 0.86, 0.67, and 0.80, Nash-Sutcliff efficiency (NSE) values of 0.52, 0.83, 0.40, and 0.57, and percent bias (PBIAS) values of 23.4, −8.5, 23.4, and 17.5 in the four different subbasins. In the validation, the statistics also indicated satisfactory results. The output of this study can be used in agricultural water resource management with irrigation practices and further in the assessment of climate change effects on the water resources in the BRB.

Hydrological Modelling of Small Gauged and Ungauged Mountainous Watersheds Using SWAT—A Case of Western Ghats in India

Mountainous forested watersheds are important hydrologic systems that are responsible for much of the water supply and run-of-the-river hy-dropower schemes in many parts of the world. In India, the Western Ghats are one of such important hydrologic systems located in southern peninsular region. Several of these watersheds are ungauged. The Soil and Water Assessment Tool (SWAT) has been used to model streamflows for two mountainous forested watersheds, namely, Gurupur (699 km2) (a gauged watershed) and Upper Payaswini (44.6 km2) (an ungauged wa-tershed). Model calibration and validation are performed using monthly and daily streamflow data for the gauged watershed. Sample flow values obtained over a limited period were used for validation of ungauged wa-tershed. Flow duration curves (FDCs) have been derived to assess per-centile flow distributions. Model performance is evaluated using Nash-Sutcliffe coefficient (ENS), percent bias (PBIAS), coefficient of de-termination (R2) and comparison of percentile flow values obtained from observed and simulated FDCs. Sensitivity analysis with Latin Hypercube One-factor-At-a-Time (LH-OAT) indicates five soil-land use related pa-rameters namely, soil available water capacity (SOL_AWC), soil evapora-tion compensation factor (ESCO), soil depth (SOL_Z) and layers, groundwater baseflow (ALPHA_BF), and curve number (CN2 (forest & agriculture)), to be sensitive for simulating both gauged and ungauged wet mountainous forested watersheds. Study shows that lateral flows from dynamic sub-surface zones in such watersheds contribute substan-tially to the total water yield.

Characterising groundwater-surface water connectivity in the lower Gandak catchment, a barrage regulated biodiversity hotspot in the mid-Gangetic basin

The alluvial aquifer system of the Indo-Gangetic Basin (IGB) is one of the world’s most important freshwater resources, sustaining humans and river ecosystems. Understanding groundwater recharge processes and connections to meteoric and surface water is necessary for effective water resource management for human and wider ecological requirements. Parts of the mid-Gangetic Basin, across eastern Uttar Pradesh and Bihar, are characterised by stable long-term groundwater levels, high annual rainfall, and limited historical groundwater use compared to parts of Northwest India for example. In this paper we use a combination of environmental tracers and hydrograph observations to characterise sources of recharge and groundwater-surface water interaction using a transect approach across the catchment of the River Gandak, a major barrage-regulated tributary of the River Ganga. Stable isotope results show that the dominant source of groundwater recharge, in the shallow (0–40 m bgl) Holocene and underlying Pleistocene aquifer system (>40 m bgl), is local rainfall. The shallow Holocene aquifer is also supplemented by local recharge from river and canal seepage and irrigation return flow in the upper and mid parts of the catchment. These observations are corroborated by evidence from detailed groundwater hydrographs and salinity observations, indicating localised canal, river and lake connectivity to groundwater. In the middle and lower catchment, river discharge is dominated by groundwater baseflow during the peak dry season when barrage gates are closed, which contributes to ecological flows for endangered river dolphins and gharial crocodiles. Groundwater residence time tracers indicate active modern recharge in the shallow alluvial aquifer system across the catchment. In the shallow Holocene aquifer elevated arsenic (As), iron (Fe), and manganese (Mn) exceeded WHO drinking water guidelines in a minority of sites, and uranium (U) and fluoride (F) concentrations approach but do not exceed the WHO guideline values. These observations varied across the catchment with higher As, Fe and Mn in the upper and mid catchments and higher U in the lower catchment. Groundwater salinity was typically between 500 and 1000 μS/cm, and isolated higher salinity was due to recharge from flood-plain wetlands and lakes impacted by evaporation. At present, the Gandak catchment has relatively high rainfall and low abstraction, which maintains stable groundwater levels and thus baseflow to the river in the dry season. Potential future threats to groundwater resources, and therefore river ecology due to the sensitivity to changes in baseflow in the catchment, would likely be driven by reductions in local monsoon rainfall, changes in water management practices and increased groundwater use.

Investigating Groundwater Discharge into a Major River under Low Flow Conditions Based on a Radon Mass Balance Supported by Tritium Data

The potentially detrimental impact of groundwater discharge into rivers on the ecosystem services provided by the river makes the localization of groundwater discharge areas as well as the quantification of the associated mass fluxes an issue of major interest. However, localizing groundwater discharge zones and evaluating their impact are challenging tasks because of (i) the limited number of suitable tracers and (ii) the high spatio-temporal variability of groundwater/river water interaction in general. In this study, we applied the ubiquitous naturally occurring radioactive noble gas radon (222Rn) as an aqueous tracer to localize and quantify groundwater discharge along a 60 km reach of the upper German part of the major river Elbe under drought conditions. All radon data processing was executed with the numerical implicit finite element model FINIFLUX, a radon mass balance-based approach, which has been developed specifically to quantify the groundwater flux into rivers. The model results were compared to the tritium (3H) distribution pattern in the studied river reach. The results of the study proved the applicability of both (i) the methodical approach (i.e., radon as tracer) and (ii) the application of FINIFLUX to drought conditions (with river discharge rates as low as 82 m3/s vs. a long time mean of 300 m3/s). Applying the model, the recorded dataset allowed differentiating between groundwater baseflow, on the one hand, and interflow and surface water runoff distributions to the river, on the other. Furthermore, the model results allowed assessing the location and the intensity of groundwater discharge into the river under low flow conditions. It was also shown that analysing discrete river water samples taken from distinct points in a major stream might lead to slightly incorrect results because of an incomplete mixing of river water and locally discharging groundwater. An integrating sampling approach (as applied for radon) is preferable here.

Modeling the Impact of Climate Change on Water Availability in the Zarrine River Basin and Inflow to the Boukan Dam, Iran

The impacts of climate change on the water availability of Zarrine River Basin (ZRB), the headwater of Lake Urmia, in western Iran, with the Boukan Dam, are simulated under various climate scenarios up to year 2029, using the SWAT hydrological model. The latter is driven by meteorological variables predicted from MPI-ESM-LR-GCM (precipitation) and CanESM2-GCM (temperature) GCM models with RCP 2.6, RCP 4.5 and RCP 8.5 climate scenarios, and downscaled with Quantile Mapping (QM) bias-correction and SDSM, respectively. From two variants of QM employed, the Empirical-CDF-QM model decreased the biases of raw GCM- precipitation predictors particularly strongly. SWAT was then calibrated and validated with historical (1981–2011) ZR-streamflow, using the SWAT-CUP model. The subsequent SWAT-simulations for the future period 2012–2029 indicate that the predicted climate change for all RCPs will lead to a reduction of the inflow to Boukan Dam as well as of the overall water yield of ZRB, mainly due to a 23–35% future precipitation reduction, with a concomitant reduction of the groundwater baseflow to the main channel. Nevertheless, the future runoff-coefficient shows a 3%, 2% and 1% increase, as the −2% to −26% decrease of the surface runoff is overcompensated by the named precipitation decrease. In summary, based on these predictions, together with the expecting increase of demands due to the agricultural and other developments, the ZRB is likely to face a water shortage in the near future as the water yield will decrease by −17% to −39%, unless some adaptation plans are implemented for a better management of water resources.

Groundwater depletion causing reduction of baseflow triggering Ganges river summer drying

In summer (pre-monsoon) of recent years, low water level among the last few decades, has been observed in several lower Indian reaches of the Ganges (or Ganga) river (with estimated river water level depletion rates at the range of −0.5 to −38.1 cm/year between summers of 1999 and 2013 in the studied reaches). Here, we show this Ganges river depletion is related to groundwater baseflow reduction caused by ongoing observed groundwater storage depletion in the adjoining Gangetic aquifers (Ganges basin, −0.30 ± 0.07 cm/year or −2.39 ± 0.56 km3/year). Our estimates show, 2016-baseflow amount (~1.0 × 106 m3/d) has reduced by ~59%, from the beginning of the irrigation-pumping age of 1970s (2.4 × 106 m3/d) in some of the lower reaches. The net Ganges river water reduction could jeopardize domestic water supply, irrigation water requirements, river transport, ecology etc. of densely populated northern Indian plains. River water reduction has direct impact on food production indicating vulnerability to more than 100 million of the population residing in the region. The results of this study could be used to decipher the groundwater-linked river water depletion as well as the regional water security in other densely populated parts of the globe.

Watershed model identifies historic wastewater discharge to wetlands as major phosphorus source to a eutrophic lake, NH USA

ABSTRACTBalke R, Koning C. O. and Lundsted B 2018. Watershed model identifies historic wastewater discharge to wetlands as major phosphorus source to a eutrophic lake, NH USA. 34:349–364.To identify the best methods of reducing phosphorus to an impaired lake, a spatially explicit, static model was developed for a shallow lake in southwestern New Hampshire. The model results indicate that (1) phosphorus concentrations in base-flow from wetland-dominated parts of the watershed were higher than expected; (2) inputs from one particular wetland complex comprised 18% of the TP inputs, representing residual phosphorus loads from a historical wastewater treatment discharge; (3) subwatershed stormwater runoff and groundwater baseflow contributed 53% of the TP inputs. Total phosphorus (TP) loadings to Pearly Pond were assessed using the Lake Loading Response Model (LLRM), which estimates TP loading and hydrologic inputs from a watershed using land use export coefficients and predicts in-lake concentrations of TP, chlorophyll a, Secchi disk transparency (SDT), and algal bloom probability through a series of empirical models. Attenuation factors were used to adjust hydrologic and pollutant load estimates to reach close agreement between predicted and observed values. This model was used to estimate pre-development phosphorus loads to the pond and resulting in-pond concentrations, and to establish the target water quality goal for the pond. The water quality goal considered the high background concentrations of phosphorus coming from undeveloped areas of the watershed, which may reflect impoundments of natural wetlands by beavers. The most cost-effective best management practices focused on the historical wastewater discharge from wetlands, waterfowl control, and education. The total cost of meeting the lake water quality goal was estimated to be $1,060,700–1,639,400.

Hydrology of the North Klondike River: carbon export, water balance and inter-annual climate influences within a sub-alpine permafrost catchment

ABSTRACT Arctic and sub-arctic watersheds are undergoing significant changes due to recent climate warming and degrading permafrost, engendering enhanced monitoring of arctic rivers. Smaller catchments provide understanding of discharge, solute flux and groundwater recharge at the process level that contributes to an understanding of how larger arctic watersheds are responding to climate change. The North Klondike River, located in west central Yukon, is a sub-alpine permafrost catchment, which maintains an active hydrological monitoring station with a record of >40 years. In addition to being able to monitor intra-annual variability, this data set allows for more complex analysis of streamflow records. Streamflow data, geochemistry and stable isotope data for 2014 show a groundwater-dominated system, predominantly recharged during periods of snowmelt. Radiocarbon is shown to be a valuable tracer of soil zone recharge processes and carbon sources. Winter groundwater baseflow contributes 20 % of total annual discharge, and accounts for up to 50 % of total river discharge during the spring and summer months. Although total stream discharge remains unchanged, mean annual groundwater baseflow has increased over the 40-year monitoring period. Wavelet analysis reveals a catchment that responds to El Niño and longer solar cycles, as well as climatic shifts such as the Pacific Decadal Oscillation. Dedicated to Professor Peter Fritz on the occasion of his 80th birthday

Modeling Streamflow in a Snow-Dominated Forest Watershed Using the Water Erosion Prediction Project (WEPP) Model

Abstract. The Water Erosion Prediction Project (WEPP) model was originally developed for hillslope and small watershed applications. Recent improvements to WEPP have led to enhanced computations for deep percolation, subsurface lateral flow, and frozen soil. In addition, the incorporation of channel routing has made the WEPP model well suited for large watersheds with perennial flows. However, WEPP is still limited in modeling forested watersheds where groundwater baseflow is substantial. The objectives of this study were to (1) incorporate nonlinear algorithms into WEPP (v2012.8) for estimating groundwater baseflow, (2) auto-calibrate the current and modified WEPP model using a model-independent parameter estimation tool, and (3) evaluate and compare the performance of the current version of WEPP without baseflow (WEPP-Cur) and the modified WEPP model with baseflow (WEPP-Mod) in simulating the hydrology of a snow-dominated watershed in the U.S. Pacific Northwest. A subwatershed of the Upper Cedar River Watershed in western Washington State was chosen for WEPP application and assessment. Simulations were conducted for two periods: 1997-2003 to calibrate the model and 2004-2011 to assess the model performance. The WEPP-Cur simulations resulted in Nash-Sutcliffe efficiency (NSE) and deviation of runoff volume (Dv) values of 0.55 and 24%, respectively, for the calibration period, and 0.60 and 21%, respectively, for the assessment period. The WEPP-Mod simulated streamflow showed improved agreement with observed streamflow, with NSE and Dv values of 0.76 and 6%, respectively, for the calibration period, and 0.74 and 2%, respectively, for the assessment period. The WEPP-Mod model reproduced hydrograph recessions during the low-flow periods and the general trend of the hydrographs, demonstrating its applicability to a watershed where groundwater baseflow was significant. The incorporation of a baseflow component into WEPP will help forest managers to assess the alterations in hydrological processes and water yield for their forest management practices. Keywords: Baseflow, Forest watershed, Hydrological modeling, Streamflow, U.S. Pacific Northwest, WEPP.

An Inexpensive and Simple Experimental Approach for the Estimation of Solute Import into Groundwater and Subsequent Export Using Inflow/Outflow Data

In agricultural catchments where groundwater (GW) base flow discharge contributes substantially towards stream flow, the information linking GW inflow/outflow with contaminant import/export is scarce. However, this information is essential to address aquatic ecosystem health hazard/risk associated with nitrate export and subsequent loading in sensitive surface water bodies (SWB). The objectives of this study were to assess the temporal dynamics of (i) rain water inflow/outflow behaviour in three agricultural catchments in the humid tropics of far-northeast Queensland of Australia, (ii) solute import via inflow and subsequent export in outflow, and (iii) the association between GW inflow/outflow and solute import/export. Approximately 71% of the average seasonal rainfall percolated (inflow) into the porous basaltic regolith of the Johnstone River Catchment (JRC) compared with 44% into the alluvial regolith in the Mulgrave River Catchment (MRC) and 29% into the metamorphic regolith in the Tully River Catchment (TRC), respectively. The outflows from the basaltic, alluvial, and metamorphic regoliths were 56%, 36%, and 55% of the inflows, respectively. The cumulative nitrate import per season was 25 k/ha in the JRC compared with 11 kg/ha in MRC and 34 kg/ha in TRC. The corresponding exports were 24 kg/ha, 8 kg/ha 26 kg/ha in JRC, MRC, and TRC, respectively. The total dissolved solute (TDS) exports were 82%, 77%, 75%, of the corresponding imports in JRC, MRC, and TRC, respectively. Simple correlations indicated that nitrate export was positively correlated with the outflow in each one of the regolith and similar trends were observed between inflow and import. The import/export mass balance for nitrate shows that 73% to 96% of the imports were exported during the same rainy season, suggesting the potential for nitrate associated ecosystem health hazard/risk in sensitive SWB receiving the outflows.

Issues of Stream-Aquifer Interactions in Grout Lined Channels in Urban Watersheds

Grout lined storm drains are used worldwide to convey storm runoff away from urban areas. They also convey dry weather flows such as car wash runoff, lawn runoff, and other “nuisance flows” in urban centers. Areas in Southern California and elsewhere have shallow water tables where dry weather flows in grout lined storm drains consist primarily of perennial groundwater baseflows. In these areas, groundwater leaks through cracks, joints, weepholes, and intentional dewatering structures into storm drains and eventually flows into natural channels, coastal wetlands, and estuaries. The “lined” sections of these channels are not always impervious; in fact, the lining of the channels often affords excellent opportunities to collect groundwater samples at discrete points that are usually not present along unlined channels where well control is sparse or where technical or legal issues prevent installation of piezometers. Frequently, as much as 95% of the nutrient and trace element loading to urban catchments during dry weather is from seepage of groundwater that is laden with pollutants. This project presents several examples of our experiences in conducting stream/aquifer studies in lined and semi-lined channels in Southern California. Sub-projects include evolution of groundwater quality along flowpaths, detection of unwanted recharge mounds in shallow groundwater systems due to leaky water main pipes, and comparisons of water quality in lined and unlined sections of a creek. Tools that are utilized and described in the paper include isotopic tracers; trace element and standard inorganic constituents; and innovative sampling methods.

The seasonal fluctuations and accumulation of iodine-129 in relation to the hydrogeochemistry of the Wolf Creek Research Basin, a discontinuous permafrost watershed

The long lived radioisotope 129I is a uranium fission product, and an environmental contaminant of the nuclear age. Consequently, it can trace anthropogenic releases of 129I in watersheds, and has been identified as a potential means to distinguish water sources in discharge (Nimz, 1998). The purpose of this work was to identify the sources and mass input of 129I and trace the transport, partitioning and mass balance of 129I over time in a remote watershed. We monitored 129I and other geochemical and isotope tracers (e.g. δ14CDIC, δ13CDIC, δ2H, δ18O, etc.) in precipitation and discharge from the Wolf Creek Research Basin (WCRB), a discontinuous permafrost watershed in the Yukon Territory, Canada, and evaluated the use of 129I as a water end-member tracer. Radiocarbon and geochemical tracers of weathering show that discharge is comprised of (i) groundwater baseflow that has recharged under open system conditions, (ii) spring freshet meltwater that has derived solutes through closed-system interaction with saturated soils, and (iii) active layer drainage. The abundance of 129I and the 129I/127I ratio correlated with geochemical tracers suggests varying contributions of these three water end-members to discharge. The 129I concentration was highest at the onset of freshet, reaching 17.4×106 atoms/L, and likely reflects the lack of interaction between meltwater and organic matter at that time. This peak in 129I was followed by a decline over the summer to its lowest value. Mass balance calculations of the 129I budget show that the input to the watershed via precipitation is nearly one order of magnitude higher than the output suggesting that such arctic watersheds accumulate nearly 90% of the annual input, primarily in soil organic matter. Temporal variations in discharge 129I concentrations correlated with changes in discharge water sources suggesting that 129I is a promising hydrologic tracer, particularly when used in concert with other stable and radioisotopes.

지하철에 의한 서울특별시 광역 지하수 유동 특성

Hydrogeologic environment of the Mega City such as Seoul, suffers from rapid changes caused by urbanization, construction of underground subway or buildings, and contaminant loading by diverse anthropogenic activities. Understanding the present condition of groundwater environment and water budget is necessary to prevent natural and manmade disasters and to prepare for sustainable water resource management of urban environment. In this study, regional groundwater flow and water budget status of Seoul was analyzed using numerical simulation. Modeling result indicated that groundwater level distribution of Seoul generally followed the topography, but the significant decreases in groundwater level were observed around the subway network. Steady-state water balance analysis showed groundwater recharge by rainfall and leakage from the water supply network was about 550,495 m³/day. Surface water inflow and baseflow rate via Han River and major streams accounted for 799,689 m³/day and 1,103,906 m³/day, respectively. Groundwater usage was 60,945 m³/day, and the total groundwater leakage along the subway lines amounted to 114,746 m³/day. Modeling results revealed that the subway could decrease net groundwater baseflow by 40%. Our study result demonstrated that the subway system can have a significant influence on the groundwater environment of Seoul.

Calculating discharge of phosphorus and nitrogen with groundwater base flow to a small urban stream reach

Summary Elevated levels of nutrients, especially phosphorus, in urban streams can lead to eutrophication and general degradation of stream water quality. Contributions of phosphorus from groundwater have typically been assumed minor, though elevated concentrations have been associated with riparian areas and urban settings. The objective of this study was to investigate the importance of groundwater as a pathway for phosphorus and nitrogen input to a gaining urban stream. The stream at the 28-m study reach was 3–5 m wide and straight, flowing generally eastward, with a relatively smooth bottom of predominantly sand, with some areas of finer sediments and a few boulders. Temperature-based methods were used to estimate the groundwater flux distribution. Detailed concentration distributions in discharging groundwater were mapped using in-stream piezometers and diffusion-based peepers, and showed elevated levels of soluble reactive phosphorus (SRP) and ammonium compared to the stream (while nitrate levels were lower), especially along the south bank, where groundwater fluxes were lower and geochemically reducing conditions dominated. Field evidence suggests the ammonium may originate from nearby landfills, but that local sediments likely contribute the SRP. Ammonium and SRP mass discharges with groundwater were then estimated as the product of the respective concentration distributions and the groundwater flux distribution. These were determined as approximately 9 and 200 g d−1 for SRP and ammonium, respectively, which compares to stream mass discharges over the observed range of base flows of 20–1100 and 270–7600 g d−1, respectively. This suggests that groundwater from this small reach, and any similar areas along Dyment’s Creek, has the potential to contribute substantially to the stream nutrient concentrations.

Heterogeneous hyporheic zone dechlorination of a TCE groundwater plume discharging to an urban river reach

The typically elevated natural attenuation capacity of riverbed-hyporheic zones is expected to decrease chlorinated hydrocarbon (CHC) groundwater plume discharges to river receptors through dechlorination reactions. The aim of this study was to assess physico-chemical processes controlling field-scale variation in riverbed-hyporheic zone dechlorination of a TCE groundwater plume discharge to an urban river reach. The 50-m long pool–riffle–glide reach of the River Tame in Birmingham (UK) studied is a heterogeneous high energy river environment. The shallow riverbed was instrumented with a detailed network of multilevel samplers. Freeze coring revealed a geologically heterogeneous and poorly sorted riverbed. A chlorine number reduction approach provided a quantitative indicator of CHC dechlorination. Three sub-reaches of contrasting behaviour were identified. Greatest dechlorination occurred in the riffle sub-reach that was characterised by hyporheic zone flows, moderate sulphate concentrations and pH, anaerobic conditions, low iron, but elevated manganese concentrations with evidence of sulphate reduction. Transient hyporheic zone flows allowing input to varying riverbed depths of organic matter are anticipated to be a key control. The glide sub-reach displayed negligible dechlorination attributed to the predominant groundwater baseflow discharge condition, absence of hyporheic zone, transition to more oxic conditions and elevated sulphate concentrations expected to locally inhibit dechlorination. The tail-of-pool–riffle sub-reach exhibited patchy dechlorination that was attributed to sub-reach complexities including significant flow bypass of a low permeability, high organic matter, silty unit of high dechlorination potential. A process-based conceptual model of reach-scale dechlorination variability was developed. Key findings of practitioner relevance were: riverbed-hyporheic zone CHC dechlorination may provide only a partial, somewhat patchy barrier to CHC groundwater plume discharges to a surface water receptor; and, monitoring requirements to assess the variability in CHC attenuation within a reach are expected to be onerous. Further research on transient hyporheic zone dechlorination is recommended.