WaterUnderground

contamination

Global fossil groundwater resources—the grandkids like hanging out with the grandparents!!!

Global fossil groundwater resources—the grandkids like hanging out with the grandparents!!!

Post by Scott Jasechko, University of Calgary

Groundwater is the world’s largest family of fresh and unfrozen water, and its members range from young to old. There are toddler groundwaters recharged more recently than the year ~1960. Our earlier research showed that these modern groundwaters make up only a small share of global groundwater stocks (Ref. 1 and Water Canada).

But what of ancient ‘fossil’ groundwater—defined as groundwater that first moved under the ground more than 12,000 years ago, before the current “Holocene” time period began?

Many studies have discovered fossil groundwaters (Refs. 2-7). These ancient groundwaters may have first become isolated under the ground during one of the ice ages (~12,000 to 2.6 million years ago), or when dinosaurs wandered the planet (230 to 65 million years ago), or even before complex multicellular life evolved (e.g., more than 1 billion years ago).

Our research shows that fossil groundwaters are widespread, based on a compilation of groundwater radiocarbon, which is common in young groundwaters but less common in fossil groundwaters.

Our recent work (Ref. 8) has two main findings:

First, we show that fossil groundwater likely makes up most of the fresh and unfrozen water on planet Earth. Fossil groundwater is common at depths deeper than ~250 meters below the ground. Our finding highlights that most aquifers take a long time to be flushed, implying that most groundwater is not rejuvenated at time scales that are consistent with water management timeframes (~decades).

Second, we show that many deep well waters that are dominated by fossil groundwater also contain some modern groundwater. That is, fossil well waters are often mixed up with recent rain and snowmelt. Because some human activities pollute recent rain and snowmelt, our finding implies that deep wells are not immune to the impacts of modern-day land uses on water quality.

Back to our family analogy – our two main findings are: (i) ‘groundwater grandparents’ (i.e., fossil water) make up most of the global groundwater family (lots of grandparents, only a few grandchildren), however, (ii) groundwater youngsters (less than ~50 years in their age), are often found to hang out at deep depths with groundwater grandparents. Once in a while, youngsters may carry the consequences of bad modern habits (i.e. contamination) down to the deep depths where the groundwater grandparents live, sullying deep groundwaters once considered immune to modern contamination.

 

Fossil groundwater discharges to the surface near the Clearwater River of northeast Alberta (56.735°N 110.471°W; video of the spring https://vimeo.com/211124266)

References

1) Gleeson T, Befus K, Jasechko S, Luijendijk E, Cardenas MB (2016) The global volume and distribution of modern groundwater. Nature Geoscience, 9, 161-168. http://www.nature.com/ngeo/journal/v9/n2/full/ngeo2590.html
2) Thatcher L, Rubin M, Brown GF (1961) Dating desert groundwater. Science 134, 105-106. http://science.sciencemag.org/content/134/3472/105
3) Edmunds WM, Wright EP (1979) Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya. Journal of Hydrology 40, 215-241. www.sciencedirect.com/science/article/pii/0022169479900325
4) Phillips FM, Peeters LA, Tansey MK, Davis SN (1986). Paleoclimatic inferences from an isotopic investigation of groundwater in the central San Juan Basin, New Mexico. Quaternary Research 26, 179-193. http://www.sciencedirect.com/science/article/pii/0033589486901031
5) Remenda VH, Cherry JA, Edwards TWD (1994). Isotopic composition of old ground water from Lake Agassiz: implications for late Pleistocene climate. Science, 266, 1975-1978. science.sciencemag.org/content/266/5193/1975
6) Sturchio NC et al. (2004) One million year old groundwater in the Sahara revealed by krypton-81 and chlorine-36. Geophysical Research Letters 31, L05503. onlinelibrary.wiley.com/doi/10.1029/2003GL019234/full
7) Holland G, Sherwood Lollar B, Li L, Lacrampe-Couloume G, Slater GF, Ballentine CJ (2013) Deep fracture fluids isolated in the crust since the Precambrian era. Nature 497, 357-360. http://www.nature.com/nature/journal/v497/n7449/full/nature12127.html
8) Jasechko S, Perrone D, Befus KM, Cardenas MB, Ferguson G, Gleeson T, Luijenjijk E, McDonnell JJ, Taylor RG, Wada Y, Kirchner JW (2017) Global aquifers dominated by fossil groundwaters but wells vulnerable to modern contamination. Nature Geoscience doi:10.1038/ngeo2943.

Deep challenges: China’s ‘war on water pollution’ must tackle deep groundwater pollution pathways

Deep challenges: China’s ‘war on water pollution’ must tackle deep groundwater pollution pathways

by Matthew Currell, School of Engineering, RMIT University, Australia

As part of its recent ‘war on pollution’, the Chinese Central Government released a major policy on water pollution control and clean-up, called the ‘10-point water plan’ in 2015. The plan aims to deal once and for all with China’s chronic water quality problems. China’s water quality deficiencies became widely recognised around the turn of the millennium, following publication of seminal works by Ma Jun, Elizabeth Economy and other local and overseas environmental campaigners. It is now widely acknowledged that chronic exposure to water pollution in China has contributed to the emergence of hundreds of cancer villages, where rates of particular types of cancer that are linked to water pollution far exceed normal population-wide averages. In addition to agricultural pollution and domestic wastewater, in many regions the pollution has resulted from industries that are part of multi-national supply chains, meaning international factors have played an important role.

In a recent review paper published in Environmental Pollution, my colleague Dongmei Han and I compiled data from official Chinese government reports to provide a snapshot of the current status of water quality in China’s major river basins, coastal waters and groundwater systems, including shallow unconfined and deeper confined aquifers (Figure 1). The results are sobering, showing that despite some recent progress, about a third of China’s river monitoring stations and more than 60% of sampled groundwater wells are seriously polluted. These data agree with an internal Ministry of Water Resources report that was briefly made public in early 2016, which showed that more than 80% of the more than 2000 monitored shallow groundwater wells in northern China’s plains areas contain serious pollution and that the aquifers they monitor are unfit to supply drinking water.

Figure 1 – Status of water quality in China based on recent government statistics. a) Surface water, ranked according to the 6-class water quality classification standard. b) Groundwater, ranked using the 5-class system in 6 sub-areas of China, including shallow and deep groundwater. Overall percentages of sampled stations/wells in each water quality class are shown as large pie-charts; percentages in yellow and red on small pie-charts indicate proportion of samples in the lowest two classes (IV & V) for shallow and deep groundwater, respectively. Both maps have been overlain with the locations of known ‘cancer villages’.

In addition to the government data, we also targeted the research literature and compiled as many datasets as possible reporting concentrations of nitrate in shallow and/or deep aquifers throughout China. Compiling these data from over 70 different sources provides greater local detail about the severity of groundwater pollution (Figure 2). We chose nitrate as an ‘indicator pollutant’ because it is widely measured, easy to detect and highly water-soluble. The presence of nitrate in a sample is often an indicator that other pollutants may also be there. The results indicate that all shallow aquifers sampled contain nitrate above the typical natural background level (approximately 1 mg/L nitrate-N or 4.5 mg/L nitrate as NO3 ion), indicating some degree of pollution. Of these 36 aquifers, samples from 25 contained nitrate concentrations exceeding the US EPA maximum contaminant level (MCL) of 10 mg/L nitrate-N. Worryingly, all but one of 37 deep or karst aquifers examined contained nitrate above the background level, while 10 of these aquifers had samples above the MCL. In five of the shallow aquifers and four of the deep aquifers, median nitrate concentrations also exceeded the MCL, meaning half of all wells in the aquifer pump groundwater with nitrate levels exceeding the maximum safe level. We also compiled groundwater stable nitrogen isotope values of the nitrate where they were available. These isotope data help to identify the major sources of nitrate pollution such as chemical fertilizers, soil nitrogen, manure and domestic wastewater, as each potential source can have a unique isotope ‘signature’. Nitrogen isotopes can also provide evidence of microbes breaking down pollution; this is important when considering whether the nitrate will naturally degrade, or if engineered clean-up strategies are required.

Figure 2 – Nitrate concentrations in groundwater from major groundwater systems in China: a) Location map of the 52 study areas from which data were compiled; b) & c) Boxplot distributions of nitrate concentrations (as N) in shallow and deep groundwater throughout China. Boxplots show median, inter-quartile range and 10th and 90th percentile values. Data is compared to the United States Environmental Protection Agency maximum contaminant level (10 mg/L) and a background concentration of 1 mg/L Nitrate-N (equivalent to approximately 4.5 mg/L nitrate as NO3- ion).

Perhaps the issue of greatest concern from our review was the observation that in addition to being ubiquitous in shallow groundwater (as is perhaps expected in areas of intensive agriculture or wastewater pollution), nitrate pollution also frequently appears in deep wells (drilled to >100m below the surface) throughout China. Normally, the time taken for water to reach these confined aquifers is long, and much of the deep groundwater in China has been dated using radio-isotopes, which indicate that it was recharged thousands or tens of thousands of years before the present. The presence of nitrate above natural background levels in these groundwater bodies suggests that pollution is undergoing rapid ‘bypass flow’ (e.g. taking short-cuts) from the surface into deep aquifers.

The Chinese Ministry of Water Resources has made public statements indicating it believes that China’s deep aquifers are safe drinking water sources, isolated from surface pollution effects due to natural geological barriers (called ‘aquitards’ by hydrogeologists). However, our data call into question this assumption. A similar finding was recently made by a group at the Chinese Academy of Sciences, who conducted a geochemical survey of tap water from various sites around Beijing. Most of Beijing’s water supply plants pump from deep groundwater wells around the city. The survey found that a significant number of samples contained nitrate and other pollutants, consistent with our findings that contamination is reaching deep aquifers through short-cut pathways. The most likely explanation is that polluted water is flowing from shallow depths down preferential conduits, such as poorly constructed or badly maintained wells, and bypassing natural geological barriers (Figure 3).

It is estimated that over 4 million wells have been drilled in China’s northern plains alone since the groundwater boom of the 1960s and 1970s. However, only a fraction of these are registered with the government or maintained. Clearly, a program to identify and plug leaking and abandoned wells is needed to stop further pollution of China’s precious deep groundwater reserves.

 

Figure 3 – Mechanism by which faulty wells can allow shallow contaminants to bypass into deep aquifers, compromising water supply safety. China has millions of unregistered wells that may act in this way, and depends on deep aquifers for much of its drinking water.

We hope that our research highlights the scale of China’s water pollution challenges, and can help the public and policy makers better understand the extent and mechanisms of groundwater pollution – a problem which is causing serious human health effects. While addressing the problem of pollution in deep aquifers will be difficult, it is too important a task to ignore, as these aquifers supply drinking water to millions of Chinese people.

References & Further reading

Currell, M.J., Han, D., Chen, Z., Cartwright, I. (2012). Sustainability of groundwater usage in northern China: dependence on palaeowaters and effects on water quality, quantity and ecosystem health. Hydrological Processes 26: 4050-4066.

Currell, M.J., Han, D. (2017). The Global Drain: Why China’s water pollution problems should matter to the rest of the world. Environment: Science and Policy for Sustainable Development 59: 16-29. http://dx.doi.org/10.1080/00139157.2017.1252605

Han, D., Currell, M.J., Cao, G. (2016). Deep challenges for China’s war on water pollution. Environmental Pollution 218: 1222-1233. http://www.sciencedirect.com/science/article/pii/S0269749116310363

Peters, M., Guo Q., Strauss, H., Zhu, G. Geochemical and multiple stable isotope (N, O, S) investigation on tap and bottled water from Beijing, China. Journal of Geochemical Exploration 157: 36-51. http://www.sciencedirect.com/science/article/pii/S0375674215300030