Association of Cattle Grazing and Recreation with Water Quality of Grazing Allotments in National Forests of Northern California.

摘要

Livestock grazing allotments on public lands managed by the U.S. Forest Service provide critical forage supporting local economies. Surface waters on public lands provide for human recreation/consumption and critical aquatic habitat. Concerns have been raised that public land water quality is being degraded by livestock grazing and recreation resulting in microbial and nutrient pollution which threatens human and ecological health. The purpose of this study was to quantify microbial and nutrient concentrations of stream water during the cattle grazing-recreation season on 12 allotments across 5 national forests in northern California. A total of 743 water samples and observations of environmental conditions and/or resource use activities were collected. Descriptive statistics were calculated for the entire dataset as well as stratified by allotment, resource use activity, environmental condition, and month of the grazing-recreation season. Results were compared to numerous fecal indicator bacteria (FIB) benchmark concentrations used in contemporary microbial water quality standards, maximum nutrient concentration recommendations, and background nutrient concentration estimates. Bivariate Poisson regression analysis was employed to test associations between mean FIB concentration for each allotment and environmental and cattle management factors.;Nutrient concentrations observed throughout the grazing-recreation season across the landscape were at least one order of magnitude below levels of ecological concern, and were similar to estimates for background water quality conditions in the region. Late season total N concentrations increased due to a flush of organic N induced by fall rainfall-runoff events. Elevated total P occurred during a storm event which transported sediment with adsorbed P on an allotment with volcanic soils. There was no evidence of excessive nutrient loading due to any resource use activity on this landscape. FIB concentrations peaked in August and September, coinciding with the greatest cattle/recreation activity and environmental conditions which promote microbial growth. All but the most restrictive FIB water quality benchmarks were broadly met, and currently recommended E. coli benchmarks were met by >93% of samples. Environmental conditions associated with elevated mean FIB concentrations include stagnant-low flow conditions, turbidity, and observation of cattle near a sample site at the time of collection. However, these mean concentrations were at or below USEPA recommended FIB benchmarks. Mean allotment FIB concentrations decreased with decreasing cattle stocking density. Mean allotment E. coli concentrations exceeded USEPA's 126 cfu/100ml recommendation on only one allotment which had one of the highest cattle stocking densities. Recreation was associated with the lowest FIB concentrations, with common recreation sites and observed recreation appearing to occur in locations, and at dates, with low FIB concentrations. These results indicate that cattle grazing, recreation and clean water are broadly compatible across these national forests.