The impacts of landslips on contaminant sources in steep pastoral hill country, Lake Tutira, North Island, New Zealand

Abstract

Landslide erosion in New Zealand hillcountry is a severe problem known, to affect pastoral farming and sediment loads in streams. The impacts of landslips on stream pollution are not well quantified, particularly at increasing distance from the slope failure. Landslides change the hydrological behaviour of hillside and contaminant sources by modifying the distribution of regolith, altering the slope form and interrupting the horizonation of the soil. This research was carried out within the Lake Tutira catchment, in Hawkes Bay, North Island. This area periodically experiences major rainstorms causing widespread landsliding and sediment deposition in the lake. The last such event was Cyclone Bola in 1988 which deposited 45cm of sediment in the lake. The investigated hypothesis was that landslipping alters the physical characteristics of hillsides and soils as sources of diffuse pollution to stream channels. It was also expected that landslides affect the pathways of contaminants from the hillside to the stream. The removal of soil and its redeposition lower down the slope changes the timing and pathways taken as it moves over and through the slope. It also changes the ability of the soil to store water and the time any water stays in the soil. The aim of the study was therefore to identify contaminants source areas and quantify their pathways along the hillside. The research site was a first order catchment, about 1.5 ha in size which was used for pastoral farming (with sheep and cattle). The study site was divided into surface condition classes to distinguish between disturbed and undisturbed areas. The classification was based on Preston (1996) but slightly modified to fit this research. The hydrological characteristics of the study catchment were investigated in a former study (Merz, 1997) and findings from that project were used to support the present analysis. Outflow from the catchment was monitored using a V-notch weir and precipitation was measured with a tipping bucket raingauge. Two surface and subsurface collectors were located in each different Contemporary Landsurface Condition class (undisturbed, landslide debris, and well developed colluvium). In addition, surface collectors were also used to collect water from the fresh landslide scars. All the monitoring equipment was attached to a datalogger that recorded all changes of the water flow. Disturbed sites were found to yield more water than undisturbed sites as a result of less storage capacity and altered topography. Contaminant concentrations in the water were found to be affected by the time the water is in touch with the soil particles, which is mainly controlled by the water holding capacity of the soil profile. The longer the water stays in the soil the greater the opportunity for chemicals to leach out and as a result higher contaminant concentrations. The chemical properties of the soil were analysed to define the sources of contaminants. Water samples from each monitoring site were tested for the main nutrients (TKN, NH3, NO3, PO4, P total, Ca, Mg, K, Na and Cl). Water emanating from within landslide debris was found to affect the water quality of the stream to a higher extent than that from undisturbed sites. This caused by the larger volume of water coming from disturbed sites and lower storage capacity of the soil. Water ponding behind clods adsorb chemicals from the soil but is washed away in next rain event, resulting in increased contamination in the stream. Landslides were found to affect flow on the hillside and contamination of the stream as a result of lower storage capacity of the soil and altered topography of the hillside.