Spoke Too Soon? Climate Change Impacts on Groundwater Contamination

As discussed in my previous post, with a warming atmosphere, very intense rainfall events (i.e. those in the upper quantiles of the current rainfall distribution) are projected to increase (Allan and Soden, 2008; Allan et al., 2010). This increase is expected to be greatest in the tropics where the warmer starting temperatures will lead to relatively greater rises in the water-holding capacity of the atmosphere (saturation vapour pressure) and thus in the moisture content of the atmosphere, as stipulated by the Clausius-Clapeyron relation (Taylor et al., 2009).

Currently, groundwater commonly has better microbiological quality (lower pathogen content) than surface water, and is therefore more suitable for human consumption (Flynn et al., 2012). However, there is evidence that the projected increase in intense rainfall under climate change will negatively impact on the microbiological water quality of groundwater. To demonstrate this, I shall focus on Bwaise, a typical densely populated informal settlement in Kampala, Uganda (Flynn et al., 2012). Over 200 protected springs supply (partially or fully) domestic water to 60% of the low-income population of the rapidly urbanising city in sub-Saharan Africa (Howard et al., 2003). The protected springs are the preferred source of domestic water for the local population of Bwaise due to its free cost, shorter distance from dwellings, and greater annual consistency in discharge compared to the piped water supply (Kiyimba, 2006; Flynn et al., 2012).

Sporadic contamination of Groundwater in Bwaise Linked to High-intensity Rainfall

Flynn et al., (2012) undertook a 2-month high-frequency discharge and water quality (chemical and microbiological) monitoring program of groundwater at Bwaise III spring, whilst noting the surrounding rainfall events. Thermotolerant coliform bacteria (TTC) are standard bacterial indicators of faecal pollution. Figure 1a shows TTC counts in the spring’s water samples remained very low in dry periods, with sharp rises occurring during and shortly after the onset of sporadic intense rainfall. This is supported by the findings of Kulabako et al., (2007), who reported intermittent but widespread contamination of groundwater in Bwaise III from faecal sources indicated by TTCs and faecal streptococci (FS), which was especially intense during the rainy season.

Figure 1. Time series plot of themotolerant coliform bacterial (TTC) counts with daily rainfall for Bwaise III Spring, Kampala in 2004, taken from a) Flynn et al., 2012, and b) Taylor et al., 2009.

Taylor et al., (2009) found very similar results during their high-frequency monitoring of the Bwaise III protected spring. They reported ephemeral gross contamination by TTC (>10³ colony forming units per 100 ml) following less than 1 hour after rainfall events of >5 mm day^-1 (Figure 1b). Particularly intense contamination (>10⁴ colony forming units per 100 ml) was observed after rainfall events exceeding 20 mm day^-1. This is true for both sets of results (Figure 1a and 1b) which show particularly large increases during the heaviest rainfall events of 2^nd August 2004 and 15^th August 2004, when concentrations in spring water rose by at least 3 order of magnitude in less than 3 hours (Flynn et al., 2012; Taylor et al., 2009). Thus, the greater the intensity of the rainfall event, the greater the intensity of contamination, and the higher the risk of water-borne disease contraction.

As such, there is strong evidence for contamination of the groundwater of Bwaise III with TTCs during intense rainfall events, with increasing contamination occurring with increasingly heavy rainfall. Howard et al., (2003) demonstrate that high sanitary risk scores correlate significantly (99% and 95% confidence intervals) to the observed bacteriological contamination during the rainy season for the springs in Bwaise. Additionally, Tumwine et al., (2002) observed the incidence of diarrhoeal diseases to increases substantially during the rainy season in Kampala, with infections with the waterborne pathogen Enterocytozoon bienusi during double that recorded during dry seasons. Tumwine et al. (2005) show that E. bienusi and Cryptosporidium spp. are primarily responsible for persistent diarrhoea in children with AIDS. Thus contamination of groundwater supplies with heavy rainfall can give rise to water borne disease outbreaks that contribute to the 3.1% of annual deaths worldwide arising from unsafe drinking water, poor sanitation and inadequate hygiene (Ashbolt, 2004).

The Mechanism for Contamination

The TTCs, indicative of faecal bacteria, are argued to derive from inadequately contained faeces in Bwaise proximate to the protected spring (Taylor et al., 2009). This is supported by the observation of constantly high concentrations of chloride and nitrate in the spring’s discharge samples (Figure 2). These concentrations are substantially greater than those in local rainfall reported by Kulabako et al. (2007), even when accounting for the concentrating effect of soil-zone evapotranspiration (ET) on rain-fed recharge, and thus point to an alternative source of water recharging the aquifer. The most obvious source within the Bwaise spring catchment is latrine effluent, with the persistently high mineral concentrations reflecting the intense loading of sewage in the spring’s catchment (Flynn et al., 2012).

Taylor et al. (2009) argue that the faecal bacteria are transported to the spring in rapid surface and near-surface flows occurring via preferential pathways. The low infiltration capacities of soils up-gradient and immediately adjacent to the Bwaise III spring (Miret-Gaspa, 2004, cited in Flynn et al., 2012), coupled with high surface gradient and limited vegetation cover over most of the catchment, result in frequent sheet overland (surface) flow during periods of intense rainfall, which is then able to enter groundwater immediately up-gradient of the spring discharge point (Flynn et al., 2012).

Measurements of TTC levels in samples of overland flow to the spring revealed it to have extremely high concentrations ranging from 3.4 x 10⁵ to 8.4 x 10⁷ cfu/100 ml (Kulabako, 2005). The infiltration of these highly contaminated flows in the immediate areas around the spring cause a deterioration in water quality but comprise only a tiny proportion of the total recharge to the spring. This explains the observed consistency in, and thus negligible effects on, the springs discharge and nitrate/chloride concentrations throughout each episode of bacteriological contamination for both Taylor et al., (2009) and Flynn et al., (2009) (Figure 2).

Figure 2. Time series of groundwater discharge, nitrate and chloride levels for Bwaise III Spring, Kampala (Uganda) throughout July and August 2004. (Source)

Increased risk of contamination (and thus risk of diarrhoeal diseases) posed by climate change in Kampala and similar environments

To investigate the specific impacts of climate change on rainfall in Kampala, Taylor et al., (2009) underwent dynamical downscaling of climate projections in Uganda from the HadCM3 GCM using a regional climate model (PRECIS). This showed substantial increases in the frequency of heavy rainfall events producing more than 10 mm day^-1 over the 20^th century for Kampala, and a decrease in the frequency of medium-rainfall events less than 10 mm day^-1 (Figure 2). Furthermore, Mileham et al., (2009) reported that this shift in the distribution of rainfall alone accounts for a 238% increase in the estimated recharge of a catchment in south-western Uganda. Under these projections, an increase in the frequency of contamination of groundwater sources in Bwaise III can be expected.

Figure 3. Frequency distribution of rainfall events for a) historical precipitation (1965-1974) and b) dynamically downscales projections of precipitation from the HadCM3 GCM for the River Mitano Basin, Uganda (Source)

How Increasing Contamination under Climate Change can be Minimised

Despite projections of increasing extreme rainfall, observations in Bwaise III point to several human factors which exacerbate contamination of the spring. Improvement of these factors could potentially minimise the negative impacts of climate change on groundwater quality and increase its potential as an adaptive source of domestic water under climate change.

For example, the area immediate up-gradient of the spring is kept clear of polluting debris and runoff by an elevated spring box. Overland flow is diverted around the spring by the box during periods of intense rainfall, which acts to promote the short-circuiting of water contaminated by waste into preferential pathways (Flynn et al., 2012). Furthermore, the box displays localised evidence of cracking, potentially permitting wastewater surface runoff to infiltrate through cracks in the masonry into the ground immediately up-gradient of the spring (Flynn et al., 2012). Based on this model, repair and maintenance of the protective concrete box surrounding the spring’s headworks would significantly reduce risk of microbiologically contaminated surface runoff impacting on the Bwaise III spring’s water quality.

As a more general point of improvement, the absence of sewerage and wastewater disposal infrastructure in many low-income settlements across Africa means much of the population disposes of sewage through infiltration to the subsurface and thus groundwater (Flynn et al., 2012). 70% of Bwaise III’s population use shared pit latrines, and 10% have no access to sanitation (Kulabako et al., 2010). Furthermore, a large proportion of the pit latrines are of the traditional unimproved type (>80%) which do not meet the basic criteria of hygiene and accessibility for children, disabled and elderly, resulting in informal sewage disposal at the ground surface (Flynn et al., 2012). As contamination is sources from faecal wastewater, improved sanitation and containment of waste could prevent increased contamination of groundwater predicted under climate change. This is supported by Taylor et al., (2009), arguing that during the 2002-2003 cholera epidemic in Uganda, (linked to anomalously intensive rainfall during the short rains associated with the ENSO event of 2002 (Alajo et al., 2006)) the duration and mortality rates of outbreaks were greater in rural areas than in Kampala where there is comparatively better access to medical treatment, sanitation, and safe water. Thus improved community hygiene is critical in preventing increased spread of disease associated with intensifying rainfall.

A final point is that the susceptibility of groundwater to contamination by coliforms in the first place depends strongly on the geological composition and thickness of the overlying aquifer material (Swartz et al., 2003). For example, batch study results in Flynn et al., (2012) confirmed the presence of pure haematite in laterite soils contributes substantially to micro-organism attenuation and inactivation, which may serve to protect underlying groundwater under increasingly intense precipitation. Not only does this suggest that some aquifers may be more resilient to contamination under climate change, but also that depending on the mechanical strength of the soil, laterites with minor amounts of haematite show potential as a substrate for low cost water filtration in African communities (Flynn et al., 2012).