Groundwater (GW) in the Mississippi Delta has some of the highest phosphorus (P) concentrations measured in the U.S. Chemical data collected from GW and surface water (SW) sites were compared to understand factors affecting P concentrations. Spatial instability in Delta GWs indicates that P sources vary. High P measurements in shallow wells near rivers, in shallow nested wells compared to deeper nested wells, and P fluctuations in wells over time suggest that the land surface may be a greater source of P in shallow groundwater than natural geological deposits. Widespread reducing conditions in shallow GW, long-term P applications to the land surface, and shallow wells being proximal to streams are possible covarying explanatory variables. Potential SW to GW pathways of P include leaching and preferential flow paths; however, GW interactions with SW via irrigation, although unnatural, can result in P deposition on soils and later transport to SW or GW. GW tracer data indicate that irrigation return flows can exceed natural baseflow discharge to some streams in late summer. Studies are needed to confirm the degree that P is mobilized from soils and bed sediment to shallow GW and to determine how declines in GW levels resulting from irrigation affect ecological services in SW.
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