6 Case study of the Ifakara study site
6.3 Water composition (IF)
6.3.3 Water quality (IF)
Water quality was assessed with regard to human health, acceptability, and irrigation purposes. Groundwater in the study site is used for drinking water and other domestic purposes. Hence, in terms of human health and acceptability, the focus was given to groundwater. In terms of irrigation, all types of water are used and/or will be used in future. Thus, a comprehensive evaluation of all water samples is given here. Finally, the influence of agricultural and other anthropogenic influences on water quality is assessed.
6.3.3.1 Water quality related to human health (IF)
All measured health related dissolved species were compared to the respective guideline value of WHO (2011) (Tab. 4.3 p.50). The health related constituents NO3-, NO2-, Mn2+, and Al3+ were analyzed for all samples of the four snapshot samplings. The concentrations of F-, and the trace heavy metals As, Cd, Cr, Cu, Ni, Pb, and Zn were determined for all water samples during the first snapshot sampling in rainy season 2014.
In general, only NO3-, NO2-, Mn2+, and F- concentrations in water samples exceeded the guideline values of WHO (2011) (Fig. 6.28). NO3- and NO2- concentrations of groundwater, stream water, and flooding water were low. The percentage amount of detectable concentrations in relation to all samples of the four snapshot samplings was 18 % for NO3- and 3 % for NO2-. Those samples, where NO3- was detected, were taken from draw and pumping wells spread over the whole study site. However, groundwater of only two sampling points showed NO3- concentrations exceeding the guideline value. While the shallow groundwater of DW36IF was only affected by NO3- in rainy season 2014, deep groundwater of PW19IF exceeded the guideline value in all seasons except for rainy season 2015. NO2- concentrations exceeded the guideline value in water of three wells in rainy season 2015 (PW19IF, DW36IF, PW48IF). As during this season no exceeding NO3- concentrations were measured, a denitrification of NO3- to NO2- might have taken place. Redox potential and oxygen saturation of those waters were lower during rainy season compared to dry season, which hints at reducing conditions facilitating denitrification (Mitsch and Gosselink 2007). The absence of NO3- in flooding water indicates denitrification as well, confirming the water quality function of the wetland in terms of nutrient removal (Hemond and Benoit 1988).
Fig. 6.28: Stacked bar charts of the percentage amount of all samples exceeding the guideline values (WHO 2011) of ni- trate (NO3-), nitrite (NO2-), and manganese (Mn2+) for the four snapshot samplings, and of fluoride (F-) for rainy
season 2014 (IF).
Almost all water samples showed detectable concentrations of Mn2+. For the different seasons, the health related guideline value was exceeded in 12 to 36 % of all samples, with more exceeded values in dry seasons compared to rainy seasons (Fig. 6.28). Most of the samples that exceeded the guideline value were taken from pumping wells and piezometers, with the exception of one draw well and one flooding water. It was observed that water of some sampling points exceeded the guideline value during all seasons, while others only exceeded them during dry seasons.
The health related guideline value for Al3+ was not exceeded in any sample. WHO (2011) already stated that those high values usually do not occur in natural waters. Similarly, all amounts of trace heavy met- als were below or around the respective limit of detection. Determination of F- turned out to show big uncertainties due to background noise in the chromatograms. A lot of water samples showed increased amounts of F-, and two deep groundwater samples even exceeded the guideline value (PW41IF, PW46IF). The exact determination of F- concentrations in groundwater within the study site will be addressed by Wieczorek (unpublished).
As shown above, chemical water quality related to drinking water is good in groundwater and surface water of the study site. This coincides with results gained from the whole Rufiji Catchment (Ministry of Water URT 2012a). However, the health related constituents NO3-, NO2-, Mn2+, and F- exceeded the guideline values in certain amounts of samples (Fig. 6.28).
In general, health related water quality was better in the non-alluvial compared to the alluvial sediments (Fig. 6.29). Most sampling points located in the alluvial fan showed DHWQs of one, while most sampling points located in the non-alluvial sediments showed DHWQs of zero. Furthermore, two hotspots with a DHWQ of three were identified (Fig. 6.29: PW19IF, DW36IF). Only these two and one other well (PW48IF) exceeded the guideline values of NO3- or NO2-. An anthropogenic contamination is expected here. For the draw well DW36IF, NO3- concentrations were much higher in rainy seasons compared to dry seasons. This indicates a leaching of accumulated NO3- from soil to groundwater during heavy rainfall events. The pumping well PW19IF is located in Ifakara city and lies directly next to a street gutter. Although this gut- ter is composed of concrete, it might have fissures providing preferential flow paths for waste water to infiltrate into the ground. However, in wide parts of the study site, no effects of drinking water quality due to anthropogenic activities were observed. Nevertheless, it is recommended not to give groundwa- ter to small children, because few hotspots of NO3- and NO2- contamination occur. These ions lead to a production of methaemoglobin in the blood, hampering the transport of oxygen (WHO 2011). Bottle-fed infants are at greatest risk, because the intake of water in relation to body weight is high.
Fig. 6.29: Groundwater quality related to human health displayed as Degree of Health related Water Quality (DHWQ). Drinking water quality decreases from 0 (no guideline value exceeded) to 3 (guideline values of three constitu- ents exceeded). Data sources: Geological Survey of Tanganyika (1962) (geology, streams), Jätzold and Baum (1968) (alluvial fan), google.com/earth (Ifakara city) (IF).
Instead, geogenic contamination was identified as the major hazard to drinking water quality. The ele- vated amounts of Mn2+ in groundwater are alarming. Manganese is known to cause neurological symp- toms, such as muscle weakness and tremor, as well as behavioral symptoms, such as memory loss and cognitive problems (McMillan 1999). More sampling points exceeded the guideline value in dry than in
rainy seasons. Thus, it is most likely that Mn2+ was derived by weathering of manganese containing sili- cate minerals. Considerable amounts of manganese were found in rock and sediment samples of both crystalline rocks and alluvial sediments (section 6.1). Therefore, a prevention of Mn2+ contamination is not possible. In general, anthropogenic contamination (NO3-, NO2-) was higher in rainy seasons, whereas geogenic contamination (Mn2+) was higher in dry seasons. Stream water did not show any contamina- tion of the analyzed indicators. However, stream water might show higher bacteriological contamina- tions, as already outlined by Ministry of Water URT (2012a), and is thus not a good alternative to groundwater in terms of drinking water.
6.3.3.2 Water quality related to public acceptability aspects (IF)
Despite the health issues of drinking water quality, the acceptability of water in terms of domestic use was evaluated. Water in the study site is used for domestic purposes, such as laundry cleaning and sani- tary purposes. The acceptability related indicators Cl-, Na+, SO
42-, Mn2+, Fe2+, Al3+, and pH were analyzed during all four snapshot samplings (Tab. 4.4 p.51). Concentrations of Zn were only analyzed for rainy season 2014.
High concentrations of some major ions give undesirable tastes to drinking water. The Na+ taste thresh- old was exceeded in deep groundwater of PW30IF during all four seasons and in groundwater of two additional pumping wells in rainy season 2014 (PW41IF, PW46IF). Furthermore, PW30IF appeared to be the only sampling point exceeding the Cl- taste threshold in all four seasons. SO
42- concentrations were all below the taste threshold. The amounts of Zn ranged between values lower than the limit of detec- tion and 0.2 mg/L, which is far below the taste threshold.
The acceptability thresholds of Mn2+ and Fe2+ were exceeded often (Fig. 6.30). Most of the samples that exceeded thresholds of Mn2+ and Fe2+ were groundwater samples. However, flooding water and stream water sometimes exceeded the thresholds as well. In each season, one deep or shallow groundwater sample exceeded the guideline value of Al3+ except for dry season 2014. While pH values of surface wa- ter were almost neutral in most cases, pH values of groundwater were slightly acidic throughout the study site. Around 50 % of all samples fell below the optimum range between 6.5 and 8.5.
Fig. 6.30: Stacked bar charts of the percentage amount of all samples exceeding the acceptability thresholds (WHO 2011) of manganese (Mn2+) and iron (Fe2+) for the four snapshot samplings (IF).
A few wells provide water that is not good in taste due to major ions. In the case of PW30IF, water users already mentioned the salty taste of the water (personal communication with local population 2015). Much more wells showed degraded tastes due to Mn2+ concentrations. Moreover, more than half of the wells displayed problems in terms of discoloration issues due to high Mn2+ and Fe2+ concentrations. This leads to quality degradation in terms of water used for cleaning. Another problem is induced by low pH
vales of groundwater, altering pipes and other parts of the well. This corrosion could lead to a dysfunc- tionality of the well, which was already observed for some wells during field work. Usually this leads to a decrease in water availability for local communities and, in extreme cases, can cause a complete ab- sence of fresh water for people. However, all of these water quality degradations related to public ac- ceptability are of geogenic origin and cannot be avoided, only treated.
6.3.3.3 Water quality related to irrigation purposes and agriculture (IF)
Surface and stream water are already used as irrigation water in the study site and bigger areas of dry land will be irrigated in future (Ministry of Water URT 2012b). Groundwater might be an alternative of irrigation water as well. In terms of salinity, affecting crop water availability, five groundwater samples showed a slight to moderate degree of restriction on use (DRU), while all surface water samples showed no restriction on use (Fig. 6.31). The evaluation of infiltration problems due to low salinity and high SAR revealed different results. Here, higher DRUs were observed. Particularly those wells that showed re- strictions on use due to salinity were the only ones that displayed no restriction on use in terms of infil- tration (e.g. PW19IF, PW30IF, DW36IF). All other groundwater samples and all surface water samples showed either slight to moderate or severe restrictions on use (Fig. 6.31). No difference was observed between rainy and dry seasons.
Fig. 6.31: Stacked bar charts of the percentage amount of all samples showing Degrees of Restriction on Use (Ayers and Westcot 1985) related to salinity and infiltration for the four snapshot samplings (IF).
Moderate SARs in combination with low ECs represent the major restrictions on use of water for irriga- tion. However, the guidelines were developed for arid and semi-arid climates and Ayers and Westcot (1985) state that they are too strict for regions, where more dilution due to precipitation occurs and the water level is shallower. Thus, slight to moderate restrictions on use might not be of big concern. Most stream waters and flooding water at the center showed severe restrictions on use, indicating a decrease in permeability of soil due to natural irrigation with those waters. This decrease in permeability is relat- ed to the leaching of Ca2+ from the soil and leads to a decreasing infiltration rate of irrigation water. The negative effects of irrigation water on soil in terms of infiltration should be considered when planning big irrigation schemes. For some crops, even the increased Mn2+ amounts could be toxic. All other trace elements are not high enough to be toxic in terms of irrigation.