Abstract
Biogeochemical processes that utilize dissolved organic carbon are widely thought to be responsible for the liberation of arsenic from sediments to shallow groundwater in south and southeast Asia. The accumulation of this known carcinogen to hazardously high concentrations has occurred in the primary source of drinking water in large parts of densely populated countries in this region. Both surface and sedimentary sources of organic matter have been suggested to contribute dissolved organic carbon in these aquifers. However, identification of the source of organic carbon responsible for driving arsenic release remains enigmatic and even controversial. Here, we provide the most extensive interrogation to date of the isotopic signature of ground and surface waters at a known arsenic hotspot in Cambodia. We present tritium and radiocarbon data that demonstrates that recharge through ponds and/or clay windows can transport young, surface derived organic matter into groundwater to depths of 44m under natural flow conditions. Young organic matter dominates the dissolved organic carbon pool in groundwater that is in close proximity to these surface water sources and we suggest this is likely a regional relationship. In locations distal to surface water contact, dissolved organic carbon represents a mixture of both young surface and older sedimentary derived organic matter. Ground–surface water interaction therefore strongly influences the average dissolved organic carbon age and how this is distributed spatially across the field site. Arsenic mobilization rates appear to be controlled by the age of dissolved organic matter present in these groundwaters. Arsenic concentrations in shallow groundwaters (<20m) increase by 1μg/l for every year increase in dissolved organic carbon age compared to only 0.25μg/l for every year increase in dissolved organic carbon age in deeper (>20m) groundwaters. We suggest that, while the rate of arsenic release is greatest in shallow aquifer sediments, arsenic release also occurs in deeper aquifer sediments and as such remains an important process in controlling the spatial distribution of arsenic in the groundwaters of SE Asia. Our findings suggest that any anthropogenic activities that alter the source of groundwater recharge or the timescales over which recharge takes place may also drive changes in the natural composition of dissolved organic carbon in these groundwaters. Such changes have the potential to influence both the spatial and temporal evolution of the current groundwater arsenic hazard in this region.
Highlights
The extensive contamination of groundwaters by geogenic arsenic (As) utilized by millions of people as a primary source of drinking water in South and South East Asia has long been recognised as a public health catastrophe (Chakraborty and Saha, 1987; Balga and Kaiser, 1996; Dhar et al, 1997; Smith et al, 2000; Nickson et al, 1998a; Winkel et al, 2008; Ravenscroft et al, 2009)
Spatial and lateral variability in As and dissolved redox sensitive cations and anions is discussed in detail in previous studies at this site
The data we present here demonstrate that recharge in close proximity to ponds has the potential to transport modern dissolved organic carbon (DOC) to depths of 44 m and dominate DOC in groundwater, even in the absence of groundwater pumping
Summary
The extensive contamination of groundwaters by geogenic arsenic (As) utilized by millions of people as a primary source of drinking water in South and South East Asia has long been recognised as a public health catastrophe (Chakraborty and Saha, 1987; Balga and Kaiser, 1996; Dhar et al, 1997; Smith et al, 2000; Nickson et al, 1998a; Winkel et al, 2008; Ravenscroft et al, 2009). Despite more than a decade of active research, several fundamental gaps remain in our understanding of the As release process that limit our ability to better predict locations and anthropogenic activities that risk As contamination. Such information is crucial for governments and those responsible for mitigating the effects of the current crisis and for predicting the potential for secular changes in the As hazard in the future. The primary source of As in the groundwaters of this region is the rapid weathering of As-bearing rocks in upper Himalayan catchments. Whilst the biogeochemical processes that are responsible for liberating As to groundwater are widely understood, identification of the source of OM that drives these processes remains unresolved, elusive and even controversial (Fendorf et al, 2010; Lawson et al, 2013; Neumann et al, 2014)
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