Shuai, Pin (2017-07). Fate and Transport of Nutrients and Contaminants Under the Impact of Surface Water and Groundwater Interactions. Doctoral Dissertation.
Periodic river fluctuations are common in nature. River fluctuations propagate into the riparian aquifer meters to hundreds of meters inland. They greatly enhance the mixing of geochemically distinct river water with groundwater and lead to intensive biogeochemical transformation of solutes in the hyporheic zone (HZ). Here, we use a combination of field methods and numerical simulations to investigate the effects of BS induced by river fluctuations due to both natural (e.g. tides and floods) and anthropogenic (e.g. hydropeaking) events on nutrients, (i.e. nitrogen (N)) and contaminants, (i.e. arsenic (As)) fate and transport. We carried out our study in two study sites: a dam regulated river in Austin, Texas and a tidally fluctuating river in Bangladesh. In the first study site, we developed a two-dimensional (2-D) coupled flow, reactive transport model to study the influence of dam release induced river fluctuations on N cycling within the HZ. Sensitivity analyses were conducted to quantify the effects of river amplitude, sediment hydraulic conductivity (K) and dispersivity, and ambient groundwater flow on nitrate removal efficiency. Our results demonstrated that daily river fluctuations created denitrification hot spots within the HZ that would not otherwise exist under naturally neutral or gaining conditions. In the second study site, we investigated the effects of tidal fluctuations on the formation of a permeable natural reactive barrier (PNRB) consisting of iron oxide precipitates and the implications of this for As trapping and mobilization in an aquifer high in dissolved As concentrations adjacent to the Meghna River. We first characterized the hydraulic properties of riverbank aquifer by using slug tests, pumping test and tidal methods. The characterized aquifer properties were used in a 2-D, flow and reactive transport model to simulate the spatial and temporal distributions of an PNRB in response to tidal and seasonal river stage fluctuations. Our study found that tidal and seasonal river stage fluctuations accelerate the formation of PNRB and broadened their spatial extent. This work, therefore, contributes to the understanding of the fate of several very important biogeochemical cycles (i.e. N, Fe and As) in a dynamically fluctuating river-aquifer system.