Effects of landslides on contaminant sources and transport in steep pastoral hill country
The effects of landslides on contaminant sources and transport during storms in steep pastoral hill country were evaluated for a small, first-order hillslope in the Lake Tutira catchment, New Zealand. Nitrate (NO3- - N) concentrations were significantly higher in soil water than in subsurface water in the saturated zone, in surface runoff, and in a stream draining the hillslope. Concentrations in colluvium and undisturbed areas were higher than in landslide debris, scars and the stream. Stream NO3- - N was lower due to instream processes, and was derived primarily from soil water in colluvium and undisturbed areas where livestock congregate and waste accumulates. Concentrations of dissolved reactive phosphorus were higher in streams than in other types of water, particularly greater than in runoff from scars. Colluvium and other unmonitored source areas were contributing dissolved reactive phosphorus to the stream. Landslide scars and debris were not significant sources of NO3- - N or dissolved reactive phosphorus because of the lack of soil, vegetation, livestock and waste in these areas. Soil and subsurface water had the highest levels of total dissolved solids because of the relatively long residence times and leaching of solutes. Landslide disturbed areas had the greatest dissolved solids concentrations and, with the exception of nutrients, were sources of solutes transported to the stream via subsurface pathways. During a storm NO3- - N loadings were largest from colluvium and debris subsurface discharges. Loadings of total P in runoff were much higher than those of dissolved reactive phosphorus and were greatest from debris. They were primarily composed of particulate P and were a significant source of elevated stream phosphorus values. Loading of total dissolved solids in runoff were also highest from debris, and loadings in subsurface water were greatest from debris and colluvium. Landslides can alter the sources and transport of contaminants in steep, agricultural hill country directly through physical disturbances that affect pollutant generation and movement, and indirectly by altering livestock use of area. These effects should be considered when modelling contaminant transport and managing these types of catchments.