Groundwater monitoring at the watershed scale: An evaluation of recharge and nonpoint source pollutant loading in the Clear Creek Watershed, Iowa

Abstract

Determining the groundwater contribution of nonpoint source pollution at a watershed scale is a challenging issue. In this study, we utilized a top‐down approach to characterize representative groundwater response units (GRUs) based on land use and landscape position (e.g., upland, sideslope, or floodplain) in the 275‐km2 Clear Creek Watershed, Iowa. Groundwater monitoring wells were then established along downslope transects in representative GRUs. This unique combination of top‐down/bottom‐up approaches allowed us to estimate groundwater pollutant loads at the watershed scale with minimal monitoring. For the 2015 study period, results indicated that more groundwater recharge occurred in the floodplain (404 mm) compared to the uplands or sideslopes (281 and 165 mm, respectively), irrespective of land use. Recharge in the floodplains consisted of 37% of the annual precipitation, whereas upland wells averaged 26% and sideslopes averaged 15% of the annual precipitation. Less recharge was found to occur beneath perennial grass compared to row crop and urbanized areas. Baseflow discharge accounted for 69% of the total NO3‐N exported from the Clear Creek Watershed, with row crop areas contributing approximately 95% of the annual load. Orthophosphorus (OP) yields were approximately 0.72 kg/ha beneath urban and suburban areas, three times higher than those in row crop or perennial areas. Urban and suburban areas accounted for 21.4% of groundwater orthophosphorus and chloride loads in the watershed compared to only 8.5% of the land area. Overall, the groundwater load allocation model for baseflow nutrient discharge to Clear Creek can be used to target future nonpoint source load reduction strategies at the watershed scale. The use of GRUs can pinpoint better areas of concern for controlling nutrient loads.

The Intensively Managed Landscape Critical Zone Observatory: A Scientific Testbed for Understanding Critical Zone Processes in Agroecosystems

“…A unique, event‐based sampling framework has been developed in IML‐CZO to capture the high spatial and temporal variability of water–sediment–nutrient transport processes that have been accelerated by intensive land management. The event‐based monitoring includes measurements on the landscape of water table fluctuations, enrichment ratios, and roughness as well as instream measurements of bank erosion, hysteresis, sediment sources, and sedimentation (e.g., Abban et al, 2016; Blair et al, 2018; Lee et al, 2017; Neal and Anders, 2015; Schilling et al, 2018; Wilson et al, 2012; Yu and Rhoads, 2018).…”

“…Recharge is estimated using these water table fluctuations where the water table rise in an aquifer is assumed proportional to the amount of recharge the aquifer received (there is no groundwater pumping in the watershed). A greater proportion of basin‐wide recharge occurs on the floodplains than on upland divides and side slopes, hence the predominant flux to the aquifer was vertical (Schilling, 2009; Schilling et al, 2018). Mean nutrient concentrations in the wells are then multiplied by mean annual recharge to estimate the annual nutrient loading.…”

“…Row‐crop areas are found to contribute ∼95% of the annual baseflow nitrate load exported from the watershed. In contrast, orthophosphorus and chloride loading rates to groundwater are higher in urban and suburban areas (Schilling et al, 2018).…”

Groundwater discharge to coastal streams – A significant pathway for nitrogen inputs to a hypertrophic Mediterranean coastal lagoon

“…10). As a perennial stream, groundwater is a major contributor to stream flow and an even more important contributor to DIN as a result of the high DIN concentrations in the aquifer (Adyasari et al, 2018;Exner-Kittridge et al, 2016;Schilling et al, 2018). The downstream part of the Salaison River in particular delivers 44% to 56% of the DIN inputs to the Or lagoon, even though it only covers 25% of the surface watershed.…”

Solute Fluxes Through Restored Prairie and Intensively Managed Critical Zones in Nebraska and Iowa

“…Previous research at the IML CZO in the Clear Creek Watershed in Iowa has focused on hillslope (Papanicolaou et al, 2015a(Papanicolaou et al, ,b, 2018 and stream bank (Neal and Anders, 2015;Papanicolaou et al, 2017) erosion that delivers sediments and associated nutrients directly to streams. Meanwhile, groundwater monitoring has highlighted that baseflow discharge to the connected stream accounts for the majority of nitrate loads exported by the stream, 95% of which is derived from row crop areas (Schilling et al, 2018). However, soils represent an intermediate step in transmitting such nutrients to groundwater or streams, yet direct measurements of soil and stream solute chemistry are still needed (Singh et al, 2018;Wilson et al, 2018).…”