Distribution and mass of groundwater orthophosphorus in an agricultural watershed

Authors: SCHILLING, KEITH - Iowa Geological Survey; STREETER, MATTHEW - Iowa Geological Survey; ISENHART, THOMAS - Iowa State University; BECK, WILLIAM - Iowa State University; Tomer, Mark; Cole, Kevin; Kovar, John

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2018
Publication Date: 1/12/2018
Publication URL: http://handle.nal.usda.gov/10113/5895398
Citation: Schilling, K.E., Streeter, M.T., Isenhart, T.M., Beck, W.J., Tomer, M.D., Cole, K.J., Kovar, J.L. 2018. Distribution and mass of groundwater orthophosphorus in an agricultural watershed. Science of the Total Environment. 625:1330-1340. https://doi.org/10.1016/j.scitotenv.2018.01.035.
DOI: https://doi.org/10.1016/j.scitotenv.2018.01.035

Interpretive Summary: Groundwater studies do not often consider how water quality changes across landscapes and implications of those changes on management. This study described patterns of dissolved groundwater concentrations in groundwater of Walnut Creek watershed (Iowa) over a two-year period. Phosphorus concentrations were greater in shallow groundwater of floodplains, where mass quantities added in recharge water were about three times higher than estimated for uplands. That is, floodplain areas contributed about 40% of the watershed groundwater phosphorus load while comprising only 17% of the land area. Groundwater mass balance suggested that more phosphorus was added to groundwater with recharge than was exported with stream baseflow. It would take at least eight years to flush out the phosphorus present in the watershed's groundwater system. Conservation practices focused on reducing rates of P addition to groundwater in floodplain areas may have a disproportionate water quality benefit compared to upland areas. These results are of interest to water resource managers and agricultural conservationists seeking effective strategies to address phosphorus loading issues in Midwestern watersheds.

Technical Abstract: Orthophosphorus (OP) is a mobile form of dissolved inorganic P that is commonly measured in groundwater studies, but the spatial distribution of groundwater OP across a watershed has rarely been assessed. In this study, we characterized spatial patterns of groundwater OP concentrations and loading rates within the 5218 ha Walnut Creek watershed (Iowa) over a two-year period. Using a network of 24 monitoring wells established across watershed, OP concentrations ranged from <0.01 to 0.58 mg/l in all samples and averaged 0.084±0.107 mg/l. Groundwater OP concentrations were higher in floodplains and OP mass loading rates were approximately three times higher than in uplands. Floodplain areas contributed approximately 40% of the watershed groundwater OP load while comprising 17% of the land area. We estimated that approximately 1231 kg of OP was present in floodplain groundwater and 2869 kg was present in upland groundwater within the shallow groundwater zone (0-5 m depth). Groundwater mass balance suggested that more OP was recharged to groundwater than exported with baseflow over the 2-year study period. Assuming no new inputs of OP to shallow groundwater, we estimated it would take approximately eight years to flush out the existing OP mass present in the system. Study results suggest that conservation practices focused on reducing OP loading rates in floodplain areas may have a disproportionately large water quality benefit compared to upland areas.