Non point source pollution with specific reference to the Mkabela Catchment.

Abstract

Non point source pollution (NPS) has long been the negated form of pollution within our natural systems. With an increase in the demand for quality crops and staple foods, there have been added pressures on water systems to cope with increasing NPS pollution (NPS-P). The effect and importance of scale on the assessment of NPS pollution has been identified as a pivotal component in the assessment of such pollutants, in particular the translation of processes from a field to a catchment scale. It has therefore become important to further investigate and research the processes involved in transporting and retaining pollutants at each measurement scale. A number of models have been developed for simulation catchments, however none of the suitably address the issue of NPS pollution and the translation of processes from the field through to the catchment scale. Each model researched fails to effectively address processes over varying scales, and tend to concentrate on a particular scale of observation. There is a distinct lack of a capable mechanism that assesses NPS pollution across varying scales within a catchment. The Water Research Commission (WRC) NPS-P project aims at eventually developing a successful model that addresses the issue of assessing NPS pollution across a number of different scales. This study aimed at assessing the loads of sediments and nutrients at different scales and included the establishment of a research catchment in the Mkabela Catchment outside Wartburg in KwaZulu-Natal, and the collection and interpretation of rainfall, runoff and nitrate data for a full year of sampling. The sampling provided valuable data for the calculation of pollutant masses and concentrations within the Mkabela Catchment. Non Point Sources are generally more dilute with suspended solids and nitrate in particular tending to have a high transport dependence upon summer events with a high intensity and low duration. A varying degree of scales were monitored during this study, ranging from plot to catchment scale in order to assess the varying influences on NPS Pollution (Nitrate and Suspended Solids). Monitoring was conducted through research mechanisms ranging from runoff plots at the plot scale to catchment scale flumes. It was found that scale has a varying influence on NPS pollution, with pollutant concentrations measured to be at a maximum at the field scale, with a value of 13.54mg/l of nitrate measured within the cane fields from event 3. Suspended solid values taken from within the water samples were most apparent at the plot scale, within the runoff plots, with a maximum of 2866.7mg/l measured during event 3 as well. It was evident from measurements and results obtained for each of the 10 sampled events that the main influencing factor of the nitrate concentrations and suspended solid values was the nature of the event. Summer rainfall events (high intensity and short duration) provided large overland flow volume that contributed largely towards the high concentrations of both nitrate and suspended solids, whereas the winter rainfall event (low intensity and long duration) contributed little to the concentrations of nitrate and suspended solids. In contrast to nitrate concentration, the largest nitrate loads by mass were measured during event 1 at the large catchment scale (Bridge 2), with a total cumulative load of 74.17kg nitrate estimated to have been yielded at the catchment outlet. The majority of nitrate are yielded from the agricultural lands where farming practices lead to the application of chemicals preplanting and post emergence. Suspended solids displayed a similar trend to that of nitrate, with an increasing cumulative yield measured throughout the catchment, resulting in a total 13414kg of sediment being measured at Bridge 2. It is interesting that Event 1 measured the largest cumulative loads for both nitrate and suspended solids; however it was recorded as an average intensity event (19.1mm/h) in comparison to the largest sampled intensity event of 165.9mm/h (Event 4) during the study. This may be attributed to the fact that the event coincided with the planting schedule of the sugarcane crops, and so the bare nature of the agricultural fields resulted in increased overland flow, and hence nitrate and suspended solid transportation. Data collected during all the events clearly show that the impoundment (a farm dam) acts as a water quality filter by retaining many of the nitrate pollutants when they enter the dam as channel flow. In summary, the controlling processes governing NPS-P movement varied through the differing scales, with crop size, artificial chemical application, nature of the event and timing during the year all contributing in varying manners at the differing scales. Future research within the WRC-NPS-P project should continue with sampling from the designated research points and add several more seasons of data to the already comprehensive first season of sampling. In addition, once a reasonable number of seasons have been sampled and analysed within the Mkabela Catchment, the initiation and development of an effective, representative scaled NPS-P model that addresses the movement and retardation of pollutants is necessary to be able to successfully model and predict the movement of NPS-P through catchment systems. In particular the effects of the controls afforded by such features as road crossings, wetlands and farm dams should be taken into account in the modelling of sediment and nutrient movement from field to catchment scale.