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 2nd August 2004 and 15th
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 20th
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).