Environmental Engineering Reference
In-Depth Information
4.2
Assessing groundwater quality
4.2.1 General parameters and nutrients
The groundwater within the farm area is not used for any beneficial purpose at present,
but it recharges the two rivers, surrounding the pasture. Results with respect to ground
water quality are presented in Table 9.4.
Unfortunately, it was no possible to obtain data regarding the aquifer characteristics
and to use it with respect to the pathways of pollution transport. A comparison with water
quality guidelines for different types of beneficial uses (Tables 9.4 and 9.5) has been
made, based on WHO guidelines (WHO 1996) and the South African guidelines for
domestic use, livestock watering and irrigation (DWAF 1996 a, b, c). The evaluation of
risks to environment has been done based on the Zimbabwe regulations for effluent,
runoff and seepage discharging into the environment (WWEDR 2000).
The pasture groundwater is neutral with respect to pH values with no significant
spatial variations. It should be noted that the increased pH and TDS values at the control
point could be explained with the fact that during the sampling period, the site has been
affected by an agricultural lime application, leading to higher values of both parameters.
TDS values of farm groundwater at all points show higher concentrations compared to
surface water quality. They fall in the green classification posing a low hazard to the
environment. TSS concentrations reflect the soils conditions. At the most frequently
irrigated site (BH4), which is located at the highest part of the pasture, along the road to
CWTW, the TSS is relatively low, indicating that the soil is well structured and the
release of soil particles is relatively low. At BH1 and BH3, which are wetlands, and
marshy conditions are typical throughout the year, the soil has changed its structure and a
considerable amount of particulate material is released to the groundwater.
Nitrate concentrations are generally low at all points, which indicates that the nitrate
leaching from the farm soil is limited despite the high pollution load (Table 9.1). Spatial
variations show that at the BH3, BH 4 and BH5 the nitrogen concentrations are twice
higher compared to the control point and the areas, which are not irrigated. A possible
explanation of the relatively low transport of nitrates to the groundwater could be the fact
that due to the sludge application, the larger fraction of nitrogen is in the form of organic
nitrogen and ammonia, which are more prone to fixation and are retained in the top soil
layer. This is confirmed by the soil characteristics (Table 9.2), which show high nitrogen
concentrations at BH4 and BH5. Due to the high hydraulic and pollution loading rates,
anaerobic conditions prevail and do not allow the biological transformation of these
fractions to nitrates, which are more mobile. Also, the high hydraulic loading rate,
especially during the rain season, leads to surface runoff, which transports part of the
material, accumulated on the surface, to the River. This scenario was confirmed by the
results in Chapter 8. In addition, the grass has a relatively high nitrogen uptake, and is
growing throughout the year, its height being controlled by the grazing cattle.
TP values fall under the blue classification (safe to the environment), except for BH5,
where the values are exceeding the safe limit slightly. However, the comparison with the
control point shows that the farm groundwater quality considerably exceeds the control
point values, which are similar to background quality (Hranova et.al. 2002), indicating
