Statistical Analysis of Temporal-Spatial Distribution of Infiltration and Groundwater Recharge, Controlling Vadose Zone-Groundwater Hydro-Biogeochemical Interactions.

Temporal dynamics and spatial distribution of the groundwater recharge of alluvial aquifers are key components in modeling of the biogeochemical interaction between the floodplain vadose zone and groundwater. We present results of long-term investigations carried out at the Rifle field site (a former uranium mill tailings site) adjacent to the Colorado River, where field studies have yielded insight into hydrological and biogeochemical processes in the unsaturated zone and aquifer. The infiltration rate and groundwater recharge were evaluated based on a statistical analysis of (1) meteorological time series data (1985-2017), including precipitation and snowmelt data, as well as evapotranspiration determined from semi-analytical and modeling studies, and (2) radioisotopic measurements of groundwater samples collected in monitoring groundwater wells. We found that net infiltration and groundwater recharge estimated based on long-term meteorological data are comparable with those established from strontium isotopic investigation, and the site averaged annual net infiltration of precipitation ranges from 4.7% to 18%, with a mean of approximately 10%. To better understand the impact of spatially varying infiltration and groundwater recharge on biogeochemical processes, we performed a statistical principal component analysis and clustering zonation, and analyzed the groundwater level fluctuations and the nitrate-to-chloride ratio of groundwater samples collected in the monitoring wells across the floodplain. Results demonstrate that the regional groundwater flow and snowmelt are primary drivers of the interaction between the alluvial aquifer in the underlying Rifle floodplain and the Colorado River. Although snowmelt induced infiltration events do not instantaneously impact water levels in the aquifer, redox states (e.g., nitrogen dynamics) of the aquifer are highly sensitive to the temporal and spatial variations of the groundwater recharge. This study highlights the importance of the corroborative statistical analysis for representing temporal-spatial distribution of the infiltration and groundwater recharge in mechanistic and statistical biogeochemical models.