MAPPING NATURALLY OCCURRING PHOSPHORUS CONCENTRATION IN THE SOUTHEASTERN UNITED STATES

The U.S. Geological Survey (USGS) has created a data set to characterize the contribution of naturally occurring phosphorus (P) to streams from weathering and erosion of surficial geologic materials, as part of the southeastern U.S. (SE) SPARROW (Spatially Referenced Regressions on Watershed Attributes) water-quality model implementation. SPARROW provides estimates of total nitrogen and P load in surface waters from point and non-point sources. The natural P data set will help separate the effects of naturally occurring P levels within watersheds from anthropogenic sources of P (such as commercial fertilizer applications).

The USGS National Geochemical Survey (NGS) contains existing data on geochemistry of stream channel sediment and soil. The P data for the SE are primarily from National Uranium Resource Evaluation (NURE) program data collected between 1976 and 1978, and individual State data collected between 1976 and 2005. This study used 9,190 point locations representing stream-sediment samples for mapping natural sediment P concentration (Pconc).

To create a continuous mapped distribution of Pconc, the NGS data were summarized by geologic unit using regional-scale geologic map units (GMU's). The GMU's delineate areas with similar lithochemistry based on geologic age as well as level 4 ecoregion units. Pconc values are averaged to produce an estimate of mean Pconc for each GMU.

Many of the stream sites represented in the NGS database may receive P from human-related sources, but potential bias introduced by these sites is minimized by averaging values from many sites within a GMU. To minimize bias for GMU's that contain very few (0-2) points, mean Pconc is estimated for these zones by adjusting values based on related or neighboring GMU values.

Sample locations from NGS are well distributed; however, many phosphate-mined areas in the SE are not represented. The data were supplemented with site locations of phosphate mines from the Mineral Resources Data System. Each mine is associated with the GMU where it is sited, and the maximum value of Pconc from stream sediment samples within the GMU is applied to an inferred area of influence (an 8-km radius) around each mine. In overlapping areas, the Pconc values for the mines replaced the mean Pconc values of the GMU's to create the final natural phosphorus distribution map.