6 Concluding Remarks

The exchange of surface water and groundwater is driven by the three-dimensional distribution of hydraulic gradients, and the magnitude of the anisotropy and heterogeneity of the associated earth materials surrounding the surface-water feature. Rivers, lakes and wetlands can gain and lose groundwater, and become disconnected from associated groundwater systems. River systems also exchange water by hyporheic flow where stream water moves into surrounding earth materials and returns to the stream. Exchanges can be identified at the landscape and individual feature scale using a variety of field and laboratory methods. These include water and geochemical mass balances; networks of monitoring wells, mini-piezometers, stage measurement, seepage runs, seepage meters, temperature sensors, and natural, environmental and introduced tracers.

In the broad sense, physical and geochemical groundwater exchanges between rivers, lakes and wetlands were recognized in the early twentieth century. Conceptual models of exchange processes were refined beginning in the last third of the century and intensive investigations of multi-scale exchanges with surface water systems were initiated. More detailed investigations of exchange processes and field, laboratory and modeling methodology development continue in the twenty-first century. The exchange of both natural and contaminated water with connected surface water and groundwater systems has been the focus of a number of investigations. Ideally, specific site investigations and the general literature will provide the qualitative and quantitative data sets needed to manage undisturbed (natural) exchange settings, and to design and remediate sites needing restoration and re-naturalization. However, the literature as of 2020 only partially addresses broad exchange questions such as: How much exchange is needed in a river reach, wetland or along a lake shore, to sustain desired ecological conditions? What are the exchange rates, locations and timing that support the flow, geomorphic character, and water quality of specific surface-water systems? Over-arching exchange concepts still need to be addressed. For example, guidelines are not available for defining the volume of exchange needed to support a trout population in a mountain stream or the type of vegetation required to sustain a land-based constructed wetland, nor are guidelines available to define the fluxes need at a lakeshore to support an endangered species. Development of such guidelines requires extensive study of a wide variety of river, lake and wetland exchange settings under natural and impacted conditions. The ecological literature that focuses on exchange needs to be better integrated with physical and geochemical hydrogeological research. The end goal should be to develop a more complete understanding of how groundwater exchange with rivers, lakes and wetlands supports the surface water system and how surface water exchange supports the associated groundwater systems.