GROUNDWATER AQUIFER AS A HYDROLOGIC OBSERVATORY

Although hydrologic observatories are intended for examining all interfaces (subsurface-surface-atmosphere) in the hydrologic cycle, the starting point in the design of a hydrologic observatory is usually the river basin. This approach is attractive because processes at land surface are readily observable. However, it may be argued that a groundwater aquifer may serve as an alternative starting point in designing a hydrologic observatory. The recharge and discharge zones of an aquifer are its interfaces to other components of the hydrologic cycle, while the flow paths through the aquifer may be conceptualized as a time line containing information of what has occurred in the recharge zone and what is to come in the discharge zone. Such an approach has its advantages and challenges. Because groundwater conditions change much more slowly than surface water, investigators might have greater opportunities to adjust monitoring schemes and experimental designs in response to observed trends. Nonetheless, groundwater systems are dynamic and contain flow paths of different lengths and magnitudes that are superimposed on one another and that may evolve with time. At the discharge zone, the mixture of groundwater originating at different locations and having flowed through different portions of the aquifer may strongly influence chemical and biological processes at the groundwater-surface water interface. Although subsurface monitoring at the regional scale is challenging and costly, the importance of groundwater input to rivers, lakes, and wetlands lends support to designing a number of hydrologic observatories based on groundwater aquifers.