Using dissolved silica to quantify transit times in aquifers and streams
Résumé
Inferring the time groundwater spends in the subsurface at the catchment scale is critical to assessing groundwater vulnerability to nutrient loading and to establish realistic restoration frameworks. This is especially true for intermediate transit times (a few years to a century), which are the timescales corresponding to intensive agriculture and associated eutrophication problems. However, limitations in the current suite of transit time proxies makes it unclear how climatic, geologic, and topographic controls interact to shape the intermediate transit time distributions (TTD) for a specific catchment. We developed a framework to test the utility of dissolved silica (DSi) to infer TTDs in shallow aquifers at catchment and regional scales. We hypothesized that silicate weathering could be approximated by a zero-order kinetic reaction since it is a transport-limited process. We calibrated this method with transit times derived from atmospheric tracers (chlorofluorocarbons) and found that DSi was a cheap and robust proxy for estimating transit times. We then used a non-intensive, process-based hydrological model informed by discharge and DSi data to infer mean transit times and dilution patterns in streams. We found that when mean transit time was related to catchment characteristics, “young” water flow depended on dynamic interactions between surface and groundwater flow. These results have several practical implications for the evaluation of groundwater resources and water quality.