An approach for using soil surveys to guide the placement of water quality buffers

Riparian forest buffers may function better in some locations than in others for filtering pollutants out of agricultural runoff water. A simple method was developed for using information contained in soil surveys to identify better locations for filtering sediment and dissolved pollutants from surface and groundwater flow. The method provides an estimate of how well a buffer would work in each soil map unit. The mapped results can guide managers to locations where protection and installation of buffers would yield greater water quality benefits.

"Vegetative buffers may function better for filtering agriculturalrunoff in some locations than in others because of intrinsiccharacteristics of the land on which they are placed. The objective ofthis study was to develop a method based on soil survey attributes thatcan be used to compare soil map units for how effectively a bufferinstalled in them could remove pollutants from crop field runoff. Threeseparate models were developed. The surface runoff models for sedimentand for dissolved pollutants were quantitative, based mainly on slope,soil, and rainfall factors of the Revised Universal Soil Loss Equation(RUSLE), and were calibrated using the Vegetative Filter Strip Model(VFSMOD) for a standard buffer design and field management. Thegroundwater model categorized map units by the presence or absence ofsuitably-shallow groundwater and hydric conditions for interaction withthe root zone of a buffer. The models were applied to a ~65 km² (~25 mi²)agricultural watershed in northwestern Missouri. Data acquisition,calculations, and map production utilized the Soil Survey GeographicDatabase (SSURGO). For surface runoff, soil survey-based valuescorrelated strongly with corresponding VFSMOD estimates for sediment (R² = 0.94) and dissolved pollutant trapping efficiency (R²= 0.83) for a wide range of soil, slope, and rainfall conditions. Astrong negative correlation between trapping efficiency and fieldrunoff load was indicated. Mapped results revealed large differences inbuffer capability for surface runoff across the test watershed (21 to99 percent for sediment and seven to 47 percent for dissolvedpollutants). Trapping efficiency for dissolved pollutants was muchsmaller than for sediment in every map unit. Lower values of trappingefficiency were associated with map units where runoff loads are higherand where a buffer will trap greater loads of sediment, but smallerloads of dissolved pollutants, than in units with higher values.Comparative rankings can be adjusted somewhat for site conditions thatdepart from the reference conditions, and recalibration may be desiredto better account for them. For groundwater, the confluence of hydricconditions and shallow water table occurred only in the highest reachesof the test watershed, but a buffer can also interact with groundwaterin most upland and riparian locations due to the prevalence of aseasonally shallow water table. By this approach, soil surveys may beused as a screening tool to guide planners to locations where buffersare likely to have a greater impact on water quality and away fromthose where impact is likely to be small." [SRS Description]