On the nature and implications of groundwater contamination

Full text: Groundwater contamination is the most important and growing part of the global water crisis. About half of the Earth's population of 7.5 billion uses groundwater in one way or another and, given lack of alternatives in many regions, most of the 2-3 billion people to be added in the next few decades will need groundwater for their survival. Groundwater contamination problems show up in many forms and have complexities much different from other freshwater sources such as rivers and lakes. Persistent contaminants common in groundwater include: nitrate, salt, petroleum organics, chlorinated solvents from cleaning fluids, chromium, arsenic, fire retardant chemicals, pharmaceuticals, pesticides, bacteria, viruses and more. Groundwater is an unequivocally critical component of the freshwater cycle with increasing reliance on it for drinking water, irrigation and energy. Its distribution and quality around the globe requires increased awareness of its function and the need for understanding of the quality and quantity impacts for its uses at the local and regional scales. Some forms of the contamination cause shutdown of wells resulting in loss of available water supply, other forms are amenable to water treatment but result in increased water cost. Also, there is contamination not yet understood and monitored but suspected of degrading human health. There is the impression that water pumped from wells should be pristine, meaning that it should be 'free of chemicals derived from human activities' but pristine groundwater is now rare. In the seventy or so decades since chemical-based industrial manufacturing and agricultural became dependant on chemical fertilizers and pesticides, along with effect of the added population of billions, nearly all aquifers used for drinking water have become pervaded with anthropogenic chemicals of one kind or another. Groundwater flows slowly, generally between one tenth to a few meters per day in aquifers. Hence, over thousands of days, contaminant plumes commonly expand kilometres or tens of kilometres from the sources where the contamination was released into the subsurface long ago. Many millions of anthropogenic organic chemicals have been produced over the past 100 years but the number showing up as significant contamination on land and in rivers and lakes are only in the thousands. But for groundwater the number is much smaller, generally less than a few hundred, because the processes that govern the transport and fate of chemicals in soil and groundwater provide significant propensity to strongly attenuate nearly all types of chemicals, the result being that few persist long enough or travel far enough to do harm. But those most recalcitrant to the attenuation processes are commonly the most difficult or expensive to remove by water treatment and many have been deemed by health authorities to be harmful at concentrations above a few micrograms per litre (parts per billion) or even at lower concentrations. This has resulted in government designated limits (maximum contaminant levels-MCLs) for drinking water set at levels so low that groundwater is increasingly found to be problematic. The number of organic chemicals for which MCLs are specified is increasing as the analytical capabilities for water analyses improve and the demand increases for more complete knowledge of the chemicals occurring in drinking water. Hence, at any moment when advanced analytical methods are used for the first time to examine existing public groundwater supplies, 'surprises' about 'new' contaminants in the water can arise setting off crises in water management for which anticipatory preparations are typically weak or nonexistent. The uncertainty about the chemical composition of public drinking water from wells has, in part, resulted in public fear that had spawned a huge global bottled water industry, relatively unregulated and with uncertainties of its own. Once groundwater contamination is located, the challenge becomes what to do about it. One approach is to treat the water at the point of use involving treatment at extraction wells and use the treated water. The alternative is to intervene with engineering means for 'remediation' by localizing the contamination using control wells (the pump-and-treat option) or by cleaning the aquifer by injection of chemicals and/ or bacteria. Pump-and-treat has been going on at a multitude of old industrial sites for decades without cleaning the aquifers substantially. Over the past four decades, trillions of dollars have been spent on groundwater 'remediation' in North America and Europe, mostly in response to prescriptive legislation but, measurable benefits worthy of the expense remain elusive. Typically, contaminants have entered and migrated with ease in the permeable parts of the groundwater flow system but removal or in situ destruction of the contaminant mass remains an immense challenge. This is largely because of the interplay of the processes governing contaminant transport and fate in the groundwater zone. The permeability of the different geological layers commonly varies over six or more orders of magnitude (e.g. between gravel and clay or between fractured and unfractured rock) with much faster transport in zones or layers where the permeability is higher and much delay in lower permeability zones where sorption, transformations, degradation mostly occur. In most contaminated aquifers, much of the contaminant mass in 'storage' in the system has, over many decades, diffused into the low permeability zones and this mass is most difficult to remove or destroy in situ but, left in place, it contributes to persistent contamination of the high permeability zones and the wells drawing water from the groundwater system. Each aquifer has its own combinations of land use history, geology, hydrochemistry, water use and stakeholders so standard prescriptive diagnoses and solutions typically are inadequate. To invoke cost-effective remediation requires adequate upfront site investigations for system characterization (i.e. problem diagnosis) but this is commonly perceived as too expensive, challenged by needs for sophistication of concepts and methods that are beyond conventional practices and therefore difficult to integrate in many circumstances. Hence, remediation failure is common. As the human population increases by billions, the intensity of our water use and changes in waste streams combined with changes to the Earth systems via land uses and climate should require sophistication of water systems characterization and monitoring to inform management but rarely this is the case for groundwater that is out of sight and largely out of mind. Groundwater is regulated over many distinct legislations in most countries but not unified in objectives or intent. Given that groundwater contamination is a complex and ever expanding problem (known as a "wicked problem"), a reasonable expectation would be that government policies and regulations in the advanced industrialized countries would have adopted comprehensive groundwater monitoring to support reliable predictions or prognoses of what is likely to come. The challenge persists unaddressed, perhaps because this would require multidisciplinary problem cooperation and analyses cutting across the bureaucracies and recognition that most of the benefits of good groundwater system understanding and monitoring are reaped longer term, generally beyond the short- term political cycles that drive government expenditures. This presentation will illustrate issues through examination of diverse examples from Canada, the United States, Europe, Brazil and China. (author)