Use of Wastewater for Irrigation: Public Health Concerns and Dangers to the Environment (A Case Study of Aisleby Farm)

Transcription

1 Use of Wastewater for Irrigation: Public Health Concerns and Dangers to the Environment (A Case Study of Aisleby Farm) Wonder Chikanya and Ellen Mangore* National University of Science and Technology, Department of Civil and Water Engineering P O Box AC 939, Ascot, Bulawayo, Zimbabwe Abstract This study evaluates the extent of chemical pollution due to wastewater irrigation on the groundwater in the vicinity of Aisleby and Good Hope farms in Bulawayo with special reference to the immediate and the future potential of the groundwater to augment public water supplies in Bulawayo. Historical data ( ) on the changes in the chemical characteristics of the water from eleven boreholes obtained from the Criterion laboratory was analysed using the Analyse-it statistical software and the Cox-Stuart statistical method. No trend could be established in the temporal trends in all the analysed parameters using both methods at 5% significance level. However, it was concluded that though wastewater irrigation was contributing to the deterioration of groundwater more factors like the hydro-geological setting of the aquifer, septic tank effluents and the land use outside the farms could also be playing a big role. Parameters like nitrates and fluorides were found to be above the drinking water limit guidelines [10] while others like chlorides and sodium were well above their taste thresholds explaining the unpalatability observed by most of the users. Moderately high Kendall Tau rank correlation coefficients were obtained for the correlation of the average concentration of most parameters with annual rainfall. Tests were carried out to reveal the current status of eight bore holes in the area and it was concluded that chlorides, calcium and fluorides were still above permissible limits. It was concluded that the water from the area is not suitable for public water supply. Key words: groundwater, hydrogeological, irrigation, pollution, trend, wastewater Introduction The city of Bulawayo lies on the watershed between the northern and the southern catchments, in the semi-arid region 4 of Zimbawe. Characteristic of semi-arid regions, the rainfall patterns of Bulawayo are erratic and high temperatures are often encountered. Usually the periods of low rainfall are coincidental with the highest evaporation potentials. These climatic conditions render the surface based water supplies unreliable. In spite of the unreliability of the surface supplies the public water supplies of Bulawayo are mainly derived from surfaces sources. Groundwater is an alternative which can be exploited to the city s ultimate benefit. Project objectives The main objective of this work is to characterize the effects of waste water irrigation on groundwater quality in Aisleby and Good Hope farms and it environs (mainly North Trenance). It is hoped that the results and recommendations of this project will be used not only for these areas but also for other situations were wastewater irrigation is practiced. This work was not in any way intended to cause the abandonment of wastewater irrigation but to ensure that a reasonable compromise between wastewater disposal and environmental conservation is maintained. The objectives can be summarised as: Predict the groundwater quality changes by historical data analysis. Assess the potability of groundwater and detect anthropogenic effects on groundwater resources. Geology and Hydrogeology The geological formation of the Aisleby Farm area is the Umganin Formation of the Upper Greenstone. The term greenstone belt refers to the metavolcanic, metavolcanosedimentary and associated meta-intrusive rocks of Bulawayan Group sequence of Archean age in the Bulawayo area [4]. The greenstone formation is classified as

2 EASTINGS having a high groundwater development potential. Three of the boreholes right on Aiselby have depths of 60, 65, 65 metres. The depths of most of the boreholes could not be established [8]. An adjacent aquifer, Matsheumhlope Wellfield, which is of the same geological formation have boreholes of average depth 40 m. The Matsheumhlope Wellfield is a shallow and mostly unconfined aquifer [9]. Details of the aquifer underlying the study area such as size, recharge, and storage capacity and abstraction rates are not well understood. A site map of the area showing the sampling points is shown in Fig. 1. Aisleby Farm NORTH TRENANCE Sampling Point NORTHINGS Figure 1: Borehole Location Map Materials and Methods Eight sampling points were selected in North Trenance (farm environs). The boreholes right on the farm were nonfunctional. The sampling points are shown on Fig. 1. Samples were collected and analysed to assess the current groundwater quality status. The results of the tests were compared with historical data. Field interviews were conducted to investigate the groundwater usage in the area. Rainfall data from the nearest meteorological station (about 10 km north of the area) was used to assess the effects of the annual rainfall on groundwater quality. The parameters to be analysed were selected based on their effect on the acceptability of the groundwater for potable use and those pertinent to groundwater pollution by wastewater irrigation. Due to technical constraints, some of parameters were not analysed for (e.g cadmium, lead and mercury). The parameters that were analysed included: ph, electrical conductivity (EC), Calcium, chloride, fluoride, phosphate, nitrate, potassium, iron and manganese. Historical data ( ) of North Trenance groundwater obtained from the Bulawayo City Council was used to establish variations in chemical quality with total seasonal rainfall, time and distance from the farm. Analytical procedures Methods of analysis were adapted from the Standard Methods[1]. Conductivity and ph were analysed in the field [5]. The other chemical parameters were analysed in the laboratory using the ELE Paqualab photometer [11]. Data analysis Historical data from the municipal water laboratory was used to investigate trends in water quality parameters. An Excel based software Analyse-it [13] was used in the correlation of seasonal rainfall and the analytes levels in the groundwater by the calculation of a measure of correlation known as the Kendall coefficient, at 5% significance level. The Kendall Tau rank correlation test was preffered to the Cox-Stuart method since it has a higher asymptotic relative efficiency (ARE) [3]. The moving average method was used to smoothen the irregularities in the time series [2]. Results and Discussions

3 Preliminary distribution graphs revealed that the historical data do not follow a particular distribution pattern and thus non-parametric methods of analysis apply. Results of the tests to evaluate the current status of the boreholes are shown in Table 1 and the annual rainfalls in Table 2. Discussions Calcium and total hardness Calcium hardness ranged from mg/L with the majority of the observations being in the mg/L range. Total hardness ranged between mg/L. There is no health-based guideline for calcium hardness in drinking water. However the taste threshold for calcium is in the region of mg/L [12]. Calcium showed a negative correlation with rainfall with = and that for total hardness was = No trend could be observed for both calcium and total hardness at the 0.05 level of significance using both the Cox-Stuart and the Kendall Tau rank correlation tests. Manganese Manganese ranged between 0.01 and 1.26mg/L with most values lying between 0.01 and 0.21mg/L. Metarmorphic rocks are known to contain large concentrations of manga-ferrous minerals (containing manganese), hence it is likely that wastewater irrigation increased the amounts of soluble manganese leading to the elevated concentrations of the metal in the groundwater Table 1: Results of water samples collected in Jan 2005 Location/Parameter ph EC TH Ca SO 4 Cl F PO 4 3 NO 3 Na K Fe Mn R/Water 202t Byo Dr Byo Dr A/Farm b Gard Prim < Prim Nerine < Jasm < Table 2: Total annual rainfalls for the period Season 1990/91 91/92 92/93 93/94 94/95 95/96 96/97 97/98 98/99 99/00 00/01 Total rainfall/mm

4 Sodium The possible sources of sodium in the groundwater of Trenance include wastewater irrigation, septic tank effluents and the sodium rich plagioclase feldspar, which also makes up the greenstone geological formation. Sodium concentrations varied between mg/L. There is no health-based limit to sodium levels in drinking water [12] however the taste threshold usually gives a reasonable restriction to the levels. The taste thresholds usually depend upon the associated anions for example its 20mg/L, 220mg/L and 150mg/L for sodium carbonate, sodium sulphate and sodium chloride respectively [12]. No temporal trends could be obtained at the 0.05 confidence interval using both the Cox-Stuart and the Kendall Tau rank correlation tests. A sodium-rainfall tau correlation of 0.49 showed a negative correlation between rainfall and chloride. Sodium concentrations generally decreased with distance from the farms. Iron Iron levels ranged between 0.03 and 1.49mg/L. Typically iron concentrations in groundwater are below 0.5mg/L [6, 7], however the acidic nature of the groundwater (shown by ph values mostly below 7) may have led to the high levels of iron. There is no health-based limit to iron in drinking water [12]. However concentrations greater than 0.3mg/L usually cause undesirable taste in water and the staining of laundry. Iron was not analysed consistently enough to allow determinations of a trend or correlation with rainfall. Nitrates Nitrate concentration observations ranged from 0.04mg/L to 27.3mg/L. The [12] limit for nitrates in drinking water is 10mg/L. Possible sources of nitrates in the boreholes include wastewater irrigation, septic tank effluents, use of fertilisers and sludge farming occurring at the farm. Nitrate levels above this limit can be fatal to infants as it leads to methemoglobinemia. The plots of spatial variation of nitrates with distance from the farms do not show any trends revealing that the nitrate concentrations depend on more than just wastewater irrigation [8]. No temporal trend in the variation of average nitrate concentrations could be detected at the 5% confidence interval. There is a positive correlation between rainfall and nitrate which can be attributed to the intensification of agricultural activities (increase in application of nitrogen fertilisers and manure) in years of high rainfall quantities. Sulphates Sulphate concentrations varied between 16.22mg/L and 521mg/L. Sources of sulphates in Trenance include wastewater irrigation infiltration, septic tank effluents and the use of gypsum fertilizers. There is no health-based limit to sulphates in drinking water [12]. No temporal trend in the variation of average sulphate concentrations could be detected at the 5% confidence interval using both the Kendall Tau correlation test and the Cox- Stuart method. Phosphates The phosphate observations were mostly small varying between mg/L. These values are very small compared to the average phosphate concentrations of 9mg/L [8] in the wastewater effluent. This shows the good treatment capacity of the soil with respect to phosphates. Phosphates were not analysed consistently enough to attempt a sensible correlation with rainfall or to establish temporal trends. Chloride Chloride levels ranged from mg/L with most observations in the 8-209mg/L interval. There is no healthbased limit for chloride. However the upper limits are usually governed by the palatability of water since chlorides tend to impact undesirable tastes to water. Possible sources of chlorides are septic tank effluents, wastewater irrigation return flows; however the general decrease in chlorides with distance from the Aisleby and Good Hope farms seem to indicate that wastewater irrigation is responsible for the elevated chloride levels. However, no trend could be observed indicating the built up of chlorides for the period at the 5% level of significance. There is a negative association between total annual rainfall and groundwater chloride concentration. Fluorides Most of the observations for fluoride are in the 0-0.9mg/L range but some are up to 5mg/L. The upper limit of the observation is a bit high considering that the boreholes are within a greenstone formation (which is not known of consisting of fluoride bearing rocks. Levels of fluoride above 1.5mg/L can lead to fluorisis [12]. There is a weak correlation between fluoride and seasonal. No trend could be established using both the Cox-Stuart and the Kendall Tau rank correlation tests at the 5% confidence interval. Conductivity The weighted average plot for conductivity shows the same shape as that of chlorides, calcium and total hardness. The conductivity varied between mS/m, which is quite high. Conductivity generally decreased with distance from the farms indicating that wastewater irrigation is causing the increase in conductivity in the water. The correlation of conductivity and seasonal rainfall showed a moderately high negative coefficient of 0.56.

5 Conclusion and Recommendations The quality of groundwater was observed to generally deteriorate in years of low total seasonal rainfall. Nitrates and sulphates were an exception in this trend indicating that the source of these pollutants might be different to that of the other pollutants and it was presumed that the source is fertilizers or some agriculture based activity besides wastewater irrigation. Analytes like nitrates, iron, and fluorides were occasionally above the WHO drinking water standards and the levels of sulphates, chlorides, calcium and total hardness were usually well above their taste thresholds concurring with the general sentiments of the residents of Trenance who observed bad taste in the water. No temporal trends could be deduced at the 5% confidence limit using both the Cox-Stuart and the Kendall Tau rank correlation tests. The fact that no trend could be established means that more factors than just wastewater irrigation could be affecting the groundwater quality these factors are likely to include the hydro-geological setting of the aquifer, methods of sewerage disposal in Trenance and agricultural activities taking place outside the farm. Besides wastewater irrigation some factors like the use of fertilizers, the use of septic tanks and the weathering of the geological formation making up the aquifer could be affecting the groundwater quality. Future work should include the in-depth analysis of contaminant transport and the performance of the wastewater treatment plant. Acknowledgements Grateful acknowledgement is due to the National University of Science and Technology, for funding of this work. References 1. APHA, (1998), Standard Methods for the Examination of Water and Wastewater, 20 th Edition, Washington, D.C 2. Chikanya, W., (2005), Chemical Contamination of Aquifers by Wastewater Irrigation. A Case study of Aisleby Farm and its environs, Final Year Project, National University of Science and Technology, Bulawayo 3. Daniel, W. W., (1990), Applied Nonparametric Statistics, 2 nd, Brooks/Cole 4. Garson, MS, (1995), The Geology of the Bulawayo Geenstone Belt and the Surrounding Granite Terrain, ZGS 5. Hach, (1997), Water Quality Analysis Handbook, HACH Co. 6. Hem, J.D., (1970) Study and Interpretation of the Chemical Characteristics of Natural Waters, USGS 7. Mangore E., (2002), Monitoring and Evaluation of the Water Quality of Bulawayo Aquifers, MPhil Thesis, National University of Science and Technology, Bulawayo, Zimbabwe 8. Mangore E., (2005), Effects of Wastewater Irrigation on the Groundwater Quality of Aisleby Farm and its Environs, Paper Presented at the 2005 Biennial Groundwater Conference, Pretoria, SA 9. Rusinga F., (2002), Bulawayo Groundwater Resource Evaluation, Unpublished MPhil Thesis, NUST, Bulawayo 10. SAZS 560:1997, (1997), Zimbabwe Standard Specifications for Water for Domestic Supplies, SAZ, 21pp 11. The Paqualab System, (1998), Paqualab Operating Instructions, ELE International Limited 12. WHO, (1999), The Recommendations of WHO for Drinking Water 13. website :