Hydrogeological tools for characterizing groundwater discharges to surface water: advantages, disadvantages, and the importance of proper timing

Abstract

Many of the water management responsibilities of Conservation Authorities require an understanding of groundwater/surface-water (GW/SW) interactions. GW/SW interactions play a major role in the physical, chemical, and biological integrity of surface water and groundwater systems and are relevant to: source water protection; protection of groundwater dependent ecosystems; protection of wetlands, management of fisheries; assessment of the effects of land use changes on watersheds; and the prediction and management of floods. The type and amount of hydrological data needed to answer specific management questions will vary greatly. If financial or personnel resources are limited, it is a challenge to collect data at a sufficient scale and frequency to ensure a full understanding of the system. The complexity of GW/SW systems means it can be difficult to collect enough data to put it all into proper hydrological context and develop reliable conceptual models of the GW/SW interactions. Therefore, there is a need for inexpensive and cost effective methods and tools to efficiently characterize groundwater discharges and GW/SW interactions on both large and small scales. To address this problem, a multi-scale, multi-technique characterization approach is proposed, where large-scale large-area reconnaissance methods are used to quickly assess large areas and identify the largest groundwater discharges. This work is then followed by progressively smaller-scale characterization techniques (with successively higher sensitivity) to make point measurements of groundwater discharge and GW/SW interactions and quantify fluxes. The reconnaissance methods consist primarily of temperature-as-a-tracer methods (i.e., aerial thermal infrared surveys, drag probe surveys, and streambed/lakebed temperature mapping) that are very useful and relatively inexpensive methods for delineating groundwater discharges. The reconnaissance results are used to pick locations for more conventional and/or point measurement techniques (i.e., differential streamflow gauging, monitoring water levels, mini-piezometers, seepage meters, and lakebed/streambed temperature monitoring and modeling) to characterize the groundwater flow directions (discharge versus recharge) and quantify fluxes. An overview of the overall approach and a brief description of the advantages and disadvantages of each technique will be provided. This approach can obtain good spatial characterization of GW/SW interactions; however, timing of the data collection is critical. All data collected should be put into the context of (and help define) how the hydrological system changes over time, because groundwater discharge and GW/SW interactions are not constant over time. For temperature-as-a-tracer methods, timing of data collection is of paramount importance. Temperature based GW/SW interactions methods are deceptively simple and can provide excellent information; however, collecting data when there is poor or no contrast between surface and groundwater temperatures should be avoided, because it can result in useless or misleading results. If properly applied, the multi-scale, multi-method characterization approach can be a cost effective way of comprehensively characterizing groundwater discharges and GW/SW interactions and help improve decision making with respect to managing natural resources.