Constraining source attribution of methane in an alluvial aquifer with multiple recharge pathways
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Identifying the source of methane (CH4) in groundwater is often complicated due to various production, degradation and migration pathways, particularly in settings where there are multiple groundwater recharge pathways. This study demonstrates the ability to constrain the origin of CH4 within an alluvial aquifer that could be sourced from in situ microbiological production or underlying formations at depth. To characterise the hydrochemical and microbiological processes active within the alluvium, previously reported hydrochemical data (major ion chemistry and isotopic tracers (3H, 14C, 36Cl)) were interpreted in the context of CH4 and carbon dioxide (CO2) isotopic chemistry, and the microbial community composition in the groundwater. The rate of observed oxidation of CH4 within the aquifer was then characterised using a Rayleigh fractionation model. The stratification of the hydrochemical facies and microbiological community populations is interpreted to be a result of the gradational mixing of water from river leakage and floodwater recharge with water from basal artesian inflow. Within the aquifer there is a low abundance of methanogenic archaea indicating that there is limited biological potential for microbial CH4 production. Our results show that the resulting interconnection between hydrochemistry and microbial community composition affects the occurrence and oxidation of CH4 within the alluvial aquifer, constraining the source of CH4 in the groundwater to the geological formations beneath the alluvium.Exploring the interaction between precipitation, surface water, and groundwater has been a key subject of many studies dealing with water quality management. The Varaždin aquifer is an example of an area where high nitrate content in groundwater raised public concern, so it is important to understand the aquifer recharge for proper management and preservation of groundwater quality. The NW part of the Varaždin aquifer has been selected for study area, as precipitation, Drava River, accumulation lake, and groundwater interact in this area. In this study, groundwater and surface water levels, water temperature, water isotopes (2H and 18O), and chloride (Cl−) were monitored in precipitation, surface water, and groundwater during the four-year period to estimate groundwater recharge. Head contour maps were constructed based on the groundwater and surface water levels. The results show that aquifer is recharged from both Drava River and accumulation lake for all hydrological conditions–low, mean, and high groundwater levels. The monitoring results of water temperature, chloride content, and stable water isotopes were used as tracers, i.e. as an input to the mixing model for estimation of the contribution ratio from each recharge source. The calculation of mixing proportions showed that surface water is a key mechanism of groundwater recharge in the study area, with a contribution ratio ranging from 55% to 100% depending on the proximity of the observation well to surface water.
Depression-focused recharge
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Modeling Stream‐Aquifer Interactions Under Seasonal Groundwater Pumping and Managed Aquifer Recharge
In South Korea, a significant amount of groundwater is used for the heating of water-curtain insulated greenhouses during the winter dry season, which had led to problems of groundwater depletion. A managed aquifer recharge (MAR) project is currently underway with the goal of preventing such groundwater depletion in a typical cultivation area, located on an alluvial aquifer near the Nam River. In the present study, FEFLOW, a three-dimensional finite element model, was used to evaluate different strategies for MAR of the cultivation areas. A conceptual model was developed to simulate the stream-aquifer dynamics under the influence of seasonal groundwater pumping and MAR. The optimal rates and duration of MAR were assessed by analyzing the recovery of the groundwater levels and the change in the groundwater temperature. The simulation results indicate that a MAR rate of 8000 m3 /d effectively restores the groundwater level when the injection wells are located inside the groundwater depletion area. It is also demonstrated that starting the MAR before the beginning of the seasonal pumping is more effective. Riverbank filtration is preferable for securing the injection water owing to plentiful source of induced recharge from the river. Locating the pumping wells adjacent to the river where there are thick permeable layers could be a good strategy for minimizing decreases in the groundwater level and temperature.
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Lower Apalachicola-Chattahoochee-Flint (ACF) River Basin, southeastern United States (U.S.). Excessive groundwater withdrawal for irrigation from the Upper Floridan Aquifer is an important issue in the lower ACF River Basin as it has led to decline in groundwater levels as well as reduction in baseflows. Since the withdrawal is projected to further increase in the future, this study evaluated the impacts of the projected increase in irrigation on the groundwater levels as well as the stream-aquifer flux in the region. The study also identified regions that are most important for groundwater recharge. Evaluation of the geohydrologic zones identified Upland Instream Karst as the most sensitive zone for recharge into the aquifer while zones in the region where the aquifer thickness was comparatively lower and close to the land surface was generally identified as sensitive. Simulation of the projected irrigation scenario predicted a reduction in groundwater levels by as much as 2.38 m, while a general reduction was predicted in much of the model domain. Large groundwater level reductions were mostly predicted in regions where the aquifer is comparatively thinner. Evaluation of the changes in stream-aquifer flux showed that flux reduced by as much as 33 % with large reductions predicted in the Lower Flint and Kinchafoonee watersheds. This study also helped identify localized zones and stream sections most susceptible to the impacts of increase in irrigation.
MODFLOW
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Groundwater discharge
Groundwater model
Water balance
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Author(s): Uriostegui, Stephanie Haydee | Advisor(s): Clark, Jordan | Abstract: The current drought crisis in California highlights the vulnerability of freshwater resources, particularly groundwater reservoirs, which supply up to 60% of California’s water during drought years. Understanding the potential impacts of climate change on groundwater recharge and storage is critical as drought periods become more frequent in the state. Groundwater residence times provide insight into groundwater recharge and transport mechanisms and storage capacities. This study developed and evaluated a new intrinsic tracer method to quantify groundwater recharge and transport using the occurrence of the naturally-produced radioisotope sulfur-35 (35S, half-life 87.5 days) in water as dissolved sulfate (35SO4). Improvements made to established analytical techniques expand the analytical range of 35SO4, which broadens the utility of 35SO4 as a hydrologic tracer. The 35SO4 tracer method was applied to two distinct hydrologic settings: 1) high-elevation Sierra Nevada basins, and 2) low-elevations basins containing managed aquifer recharge (MAR) facilities. In the Sierra Nevada study, the new 35SO4 method was used to empirically constrain annual groundwater recharge in Sagehen Creek Basin (SCB) and Martis Valley Groundwater Basin (MVGB). Compared to relatively high 5SO4 activity in seasonal snowmelt (5.5 ± 0.3 to 52.9 ± 3.4 mBq/L), groundwater and surface water consistently yielded low 35SO4 activities resulting in a calculated percent new snowmelt (PNS) of l30%. The consistently low PNS suggests that recent (l1 year old) snowmelt represents only a small fraction of the larger aquifer system. As snowpack continues to decline due to climate change, streamflow and springs may respond in a two phase manner: rapid response in discharge followed by more gradual decreases over decades due to declines in groundwater recharge. The MAR study used 35SO4 to quantify groundwater travel times near MAR operations. MAR sites divert excess surface water, imported water, and reclaimed wastewater into surface-spreading ponds or direct injection wells to replenish groundwater in heavy-usage areas. Identifying groundwater travel times near MAR facilities is critical for determining the fate and transport of potential contaminants, especially for facilities that incorporate a significant portion of reclaimed wastewater. Successful application of the 35SO4 tracer method near MAR sites is dependent on careful characterization of the 35SO4 activity in source waters. Relative to established deliberate tracer experiments, which require extensive field and laboratory effort, the less intensive 35SO4 technique showed comparable groundwater travel times at two MAR facilities located in southern California. Both the Sierra Nevada and MAR studies demonstrate that 35SO4 is a valuable, yet underutilized tracer in hydrologic studies.
Snowmelt
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