Fecal Contamination of Pastoral Wetlands
Journal of Environmental Quality , 33 , 2004
Near-channel hill-country wetlands draining steep pastoral land in New Zealand exhibit high levels of fecal contamination at a range of flows. This contamination is attributed to both the transport of bacteria into a wetland from the surrounding catchment and the direct excretion of fecal material onto wetlands by grazing cattle. E. coli concentrations observed at low to moderate flow at 20 sites varied between 0.5 x 101 and 2 x 104 most probable number (MPN) 100 mL–1. High flow concentrations measured at two wetlands ranged up to 6 x 106 MPN 100 mL–1 and yielded storm period bacterial loads of between 1 x 106 and 3 x 1010 MPN per event. Given the disproportionately large fraction of surface and subsurface flow from the catchment that passes through the wetlands, these yields represent a large proportion of the total loss of bacteria from steep grazed hillsides, across a range of storm events. Cattle are attracted to the smaller, shallower wetlands for grazing in both summer and winter. Excluding stock from shallow wetlands may therefore yield improvements in bacterial water quality, although accurately quantifying this improvement is difficult without long-term studies. Cattle are not attracted to larger, deeper wetlands, presumably for fear of entrapment, and fencing them is unlikely to realize significant improvements in bacterial water quality. A statistical model incorporating solar radiation and flow explains 87% of the variance in E. coli concentrations across five monitored rainfall events. A positive correlation was found between solar radiation and E. coli concentration. The study was conducted in winter when clear, sunny days are relatively cold. Solar radiation on these days appears to be too weak to promote die-off but the colder temperatures aid survival.