ABSTRACT:

Layered heterogeneity in riverbank or aquifer lithology imparts a threshold effect causing nonlinear interactions between river stage and groundwater level that can be simulated as a dynamical system. A simple dynamic water-balance/linear-reservoir model was developed to investigate threshold effects at a location on the Big Sunflower River in the Lower Mississippi River Valley, USA. Four conceptual models, each of which simulates a perched aquifer as a dynamical system that receives recharge from the riverbank and loses water to an underlying regional aquifer, were tested using combinations of zero, one, or two thresholds representing layered heterogeneity in riverbank and aquifer lithology. Models were run using a 382-day period of hourly stream-gauge measurements and calibrated to corresponding measurements in a nearby well. All models matched observed groundwater levels reasonably well, with a maximum root-mean-square error (RMSE) of 0.49 m for the calibration period. Final model performance was assessed for a 3.5-year period representing varied hydrologic conditions. The heterogeneous models matched high-stage events substantially better than the homogeneous model. The best performance was by the model incorporating threshold effects (RMSE of 0.268 m for the period of record), which elucidated four modes of GW-SW system behavior controlled by both riverbank (riverbed hydraulic conductivity) and aquifer (transmissivity and storage coefficient) properties. The dynamical system modeling approach is relevant to any GW-SW system with layered heterogeneity, and the simple dynamic water-balance/linear-reservoir model has broad applicability to a wide range of hydrogeologic settings.

ABSTRACT:

Lake stage, wetland stage, and groundwater levels in 11 wells collected by Michael C. Gratzer at the University of Mississippi Department of Geology & Geological Engineering. Data used in the following with abstract below: Gratzer, M.C., Davidson, G.R., O'Reilly, A.M., and Rigby, J.R., (in press 12/2019), Groundwater recharge from an oxbow lake-wetland system in the Mississippi Alluvial Plain, Hydrological Processes, https://doi.org/10.1002/hyp.13680

The Mississippi River Valley Alluvial Aquifer ranks among the most over-drafted aquifers in the United States due to intensive irrigation. Concern over declining water levels has increased focus on understanding the sources of recharge. Numerous oxbow lakes overlie the aquifer which are often considered hydraulically disconnected from the groundwater system due to fine-grained bottom sediments. In the current study, groundwater levels in and around a 445-ha oxbow lake-wetland in Mississippi were monitored for a two-year period that included an unusually long low-water condition in the lake (>17 months), followed by a high-water event lasting over four months before returning to earlier low water levels. The high water pulse (>4 m rise) provided a unique opportunity to track the impact in the underlying alluvial aquifer. During low-water conditions, groundwater flowed westward beneath the lake. Following the lake rise, groundwater beneath and near the perimeter responded as quickly as the same day, with more delayed responses moving away from the lake. Within two months, a groundwater mound formed near the center of the oxbow (>3 m increase), with a reversal in the local hydraulic gradient toward the east. Flow returned to a westward gradient when the lake level dropped back below 0.3 m. Analysis of precipitation and nearby river stage could not account for the observed behavior. Recharge to the aquifer is attributed to rising water levels spreading over point bar deposits and into surrounding forested wetlands where preferential flow pathways are likely to exist due to buried and decomposing tree remains. An earlier study in the wetland demonstrated increasing redox potential in isolated zones, consistent with the existence of preferential flow pathways through the bottom sediments (Lahiri & Davidson, this issue). Retaining high water levels in oxbow lakes could be a relatively low-cost water-management practice for enhancing aquifer recharge.

ABSTRACT:

Model developed and documented in: O’Reilly, A.M., Holt, R.M., Davidson, G.R., Patton, A., Rigby, J.R., 2020. A dynamic water-balance/nonlinear-reservoir model of a perched phreatic aquifer–river system with hydrogeologic threshold effects: Water Resources Research 56(6): e2019WR025382. https://doi.org/10.1029/2019WR025382

Heterogeneity in the hyporheic zone or near-field geology can impart a threshold effect on groundwater-surface water (GW-SW) exchange. Variations in the texture of riverbed sediments and lithologic variations in adjacent and underlying geology are examples of common heterogeneities. Hydrologic interaction with these heterogeneities leads to distinct types of “behavior” that “switch” when surface-water or groundwater levels rise above or fall below the interface of the layers of differing lithology. A dynamic water-balance/nonlinear-reservoir model incorporating threshold effects was developed for a perched phreatic aquifer–river system. Four conceptualizations of the system were modeled, each of which simulates a perched aquifer as a dynamical system that receives recharge from the riverbank and loses water to an underlying regional aquifer, using combinations of zero, one, or two thresholds representing layered heterogeneity in riverbank and/or aquifer lithology. Application of the model code was demonstrated at a location in the Lower Mississippi River Valley, USA. Models were run using hourly river-gage measurements, calibrated to a 382-day period of corresponding measurements in a nearby well, and further assessed for a 3.5-year period representing varied hydrologic conditions. The best performance was demonstrated by the model incorporating threshold effects, which elucidated four modes of GW-SW system behavior controlled by both riverbank (riverbed hydraulic conductivity) and aquifer (hydraulic conductivity and storage coefficient) properties. The dynamical system modeling approach incorporates the salient hydrologic processes of a GW-SW system with layered heterogeneity. Based upon fundamental mass-conservation concepts, the simple dynamic water-balance/linear-reservoir model has broad applicability to many hydrogeologic settings.