Handbook of Vadose Zone Characterization & Monitoring

L. Gray Wilson, Lorne G. Everett, Stephen J. Cullen

• Editore: CRC Press
• Anno: 2018
• ISBN:
  9781351441940
• Edizione: 1
• Argomento: Technology & Engineering
• Categoria: Environmental Management

This book is written in a simple, straightforward manner without complicated mathematical derivatives. Compiled by experienced practitioners, this guide covers topics such as basic principles of vadose zone hydrology and prevalent monitoring techniques. Case studies present actual field experiences for the benefit of the reader. The Handbook provides practitioners with the information they need to fully understand the principles, advantages, and limitations of the monitoring techniques that are available.
The Handbook of Vadose Zone Characterization & Monitoring expands and consolidates the useful and succint information contained in various ASTM documents, EPA manuals, and other similar texts on the subject, making it an invaluable aid to new practioners and a useful reference for seasoned veterans in the field.

1

Is Our Ground-Water Monitoring Strategy Illogical?

Stephen J. Cullen, John H. Kramer, Lorne G. Everett, and Lawrence A. Eccles

INTRODUCTION

Traditional ground-water monitoring strategies have relied on a process of collecting samples from wells and determining if there is evidence of contamination. This approach is arguably illogical, because if the purpose of monitoring is preventing aquifer contamination, the evidence comes only after contaminants have reached the resource we are trying to protect. Monitoring in the vadose zone, on the other hand, can provide early detection of contaminant migration and the opportunity to actually prevent ground-water contamination. Because this fact has been increasingly recognized in the regulatory arena, ground-water students and professionals need training in vadose zone hydrogeology. While the relative hydrologic importance of the vadose zone is much greater in the western United States, it remains a largely misunderstood portion of the geologic profile across the country. The vadose zone, sometimes saturated but more often not, is complicated by textural heterogeneities, flow instabilities, and preferred pathways. Reliable prediction of contaminant migration is lacking. Thus, ongoing monitoring is the only viable option and development of innovative monitoring techniques should be encouraged in both the public and private sectors. For example, deployment of monitoring techniques such as neutron moderation, dielectric measurements, and soil gas screening represent efficient, low-cost methods for improving monitoring coverage in space and time.

WHAT IS GROUND-WATER MONITORING REALLY?

Attempting to prevent aquifer contamination by monitoring ground water is akin to monitoring a patient’s heartbeat to determine when a heart attack will occur. By the time the heartbeat becomes irregular or stops, the patient is typically experiencing a serious trauma. Likewise, by the time significant contamination shows up in a traditional ground-water sampling program, the ground water is already contaminated. Prudence requires attention to monitoring the parameters symptomatic of the condition. In the case of the patient, breathlessness and chest pains are symptomatic of heart problems. In the case of ground-water contamination, the vadose zone is the logical place to look for symptoms because a contaminant release to the vadose zone always precedes ground-water contamination, with the notable exception of ground water in direct hydraulic communication with surface water.

Ground-water monitoring programs have long been used to detect, observe, regulate, and control aquifer water quality. In earlier years, this approach seemed adequate because demand on ground-water supplies was lower, industrial sources of contaminants fewer, and the perceived public health risks low. In today’s technological society, agricultural and urban demands placed on ground water are explosive, and the variety of primary or secondary toxic compounds produced is steadily increasing. Improved analytic instrumentation now provides measurement capability of water sample toxics in the parts-per-billion range, while our awareness of the toxicological effects of various contaminants has widened. This has increased the chance of detecting contamination, and elevated our evaluation of the risk. Once aquifer contamination is detected, a series of undesirable and costly events are irrevocably placed in motion, driven by regulatory mandates at the federal, state, or local levels. These events can include more intensive compliance monitoring, corrective action to remove the source of contamination, aquifer remediation, and shutdown of facilities. Moreover, the National Research Council (1990) noted that “Cleanup objectives may not be achievable at all, especially with dense nonaqueous phase liquids.” For these reasons, prevention, not mere detection, is the desirable goal.

As the result of vigorous federal legislative activity, the U.S. Environmental Protection Agency (EPA) adopted a Ground Water Protection Strategy in 1984, subsequently revised in July 1991. In clear and concise language, EPA stated that “… the overall goal of EPA’s Ground Water Policy is to prevent adverse effects to human health and the environment, and to protect the environmental integrity of the nation’s ground-water resources …,” (U.S. EPA, 1991). A significant component of the EPA’s newly revised and rapidly evolving policy clearly places an increased emphasis on prevention of ground-water contamination, and on efforts to achieve a greater balance between prevention and remediation activities.

THE VADOSE ZONE

The vadose zone is that portion of the geologic profile beneath the earth’s surface and above the first principal water-bearing aquifer. Flow in the vadose zone is dynamic and characterized by periods of unsaturated flow at varying degrees of partial saturation punctuated by episodes of preferential, saturated flow in response to hydrologic events or releases of liquids. As Bouwer (1978) pointed out, it is inappropriate to refer to this zone as the “unsaturated zone” or to refer to all flow in this zone as “unsaturated flow.”

It is time to consider the possibility that we have painted ourselves into a terminology corner. The term “ground water” has historically come to mean water beneath the land surface contained in interconnected pores in the saturated zone that is under hydrostatic pressure (D.M. Nielsen, 1991; Bouwer, 1978; Freeze and Cherry, 1979; Driscoll, 1986). Unfortunately, this leads to an artificial separation in the minds of many between water above and below the water table. In fact, all subsurface water is ultimately connected hydraulically via intergranular films. Water naturally flows to and from the ground-water table in response to gravitational, pressure, and matric potential gradients. To avoid misconceptions about vadose zone flow processes, it would be preferable for all water in the subsurface environment, including vadose zone water, to be considered ground water. Since it is not practical to rewrite the textbooks, we promote the use of the term “subsurface water” as used by Driscoll (1986), and remind readers that ground water is a component of the larger category of subsurface water, which also includes hydraulically connected vadose zone water.

WHY VADOSE ZONE MONITORING?

The vadose zone is an intrinsically complicated three-phase system consisting of interconnected matrices of solid, liquid, and gas phases and is a complex environment in which to characterize and predict water flow. Vadose zone contaminant flow is confounded by partitioning into nonaqueous, dissolved, gaseous, and sorbed phases. Downward saturated flow through the “filter” of a porous medium driven primarily by gravity and hydrostatic head, as described by Darcy and refined by others (Green and Ampt, 1911), is intuitive. Such flow is sometimes diverted in the vadose zone by barriers causing lateral transport, or accentuated by preferred pathways promoting rapid downward transport (Germann, 1988; Glass et al., 1988). Less intuitive is unsaturated flow driven by the attraction of water for soil particle surfaces. Matric forces can be stronger than gravity and result in water flowing uphill! Predictive modeling of such a complex environment is, at best, frustrating. Nielsen et al. (1990), reviewing the state-of-the-art of predicting contaminant transport in the vadose zone, stated that, “The efficacy of accurately predicting the attenuation and eventual location of solutes or constituents in the vadose zone remains undeveloped.” They further concluded, “Because the reliability of models for contaminant transport has not been established even for site-specific conditions, it appears that direct monitoring of the constituents in the vadose zone remains a necessity into the foreseeable future.”

Beyond the pure logical value of detecting contamination before it reaches ground-water supplies, vadose zone monitoring can decrease the expense and technical difficulties associated with ground-water remediation. In the event of a release, early detection by vadose zone monitoring can result in faster and less costly remediation because of the generally reduced contaminant concentrations, reduced volume of contaminated subsurface material, and higher levels of oxygen to promote degradation and/or recovery.

VADOSE ZONE REGULATORY EVOLUTION

Since the National Environmental Policy Act became law in 1969, society’s collective consciousness has been raised regarding the important role the vadose zone plays as a buffer against ground-water contamination. Just after its formation in 1970, the EPA funded the first major national ground-water monitoring study involving numerous noted hydrogeologists such as Banks, Geraghty, Schmidt, Tinlin, Todd, Warner, and others, which was later published by Everett (1980). Supported by the EPA’s Environmental Monitoring Systems Laboratory (EMSL) at Las Vegas, this initial investigation recognized the need for early alert or vadose zone monitoring as part of an overall ground-water monitoring strategy. Wilson (1980) first described approaches to monitoring the vadose zone in a report to the EPA.

The Resource Conservation and Recovery Act (RCRA), passed in 1976, identified land treatment as a nationally permitted method for hazardous waste disposal. Because land treatment operates as an open system, vadose zone monitoring was required within 30 cm of the bottom of the treatment zone. As a result of the first federally mandated regulations (45 FR 33248 and 47 FR 32363) covering the vadose zone, Permit Guidance Manual on Unsaturated Zone Monitoring for Hazardous Waste Land Treatment Units (U.S. EPA, 1986), was published by EMSL-Las Vegas. This manual, based on a report to the EPA by Everett and Wilson (1986), has since been widely referenced in other federal and state legislative applications.

The RCRA was amended in 1984 and the EPA, under Subtitle I, required vadose zone monitoring at Underground Storage Tanks (UST) sites where sufficient depth to the water table existed. Subsequently, numerous state approaches required vadose zone monitoring when water table depth was shallower than that dictated by the federal position. Thus, vadose zone monitoring has been accepted nationally as a viable part of a monitoring strategy.

In 1974, Congress enacted the Safe Drinking Water Act (public law 93–523) including provisions to protect existing and future underground sources of drinking water. Under Part C of the act, the EPA developed regulations to protect underground sources of drinking water from contamination by the subsurface injection or emplacement of fluid through wells. Although this act allowed Class V wells (dry wells or vadose zone wells) to inject nonhazardous fluids into or above underground sources of drinking water, some states and regions have recognized that this practice may be hazardous. In fact, some have discontinued the use of dry wells associated with gas stations and industrial facilities. Clearly, the use of Class V wells associated with sites where petroleum or chlorinated hydrocarbons are used should be discontinued nationally. Procedures to close and remediate these Class V wells are in preparation (Everett, 1992).

In 1994, amendments to the RCRA are under consideration. Fundamental to the reauthorization will be the inclusion that vadose zone monitoring can be required as part of Subtitle C activities related to Part B permits at hazardous waste treatment, storage, and disposal facilities. Based on the decision of EPA Regional Administrators, vadose zone monitoring will therefore be a key component of our national hazardous waste disposal monitoring program.

A clear indication of regulatory changes can be seen in California. Title 14, Chapter 5 of the California Code of Regulations (CCR) requires that reports from every landfill (operating or closed) contain a chemical characterization of the soil-pore liquid from areas likely to be affected by leachate leaks. In addition, under CCR Title 23, Chapter 15, California requires that every operating and closed landfill have a vadose zone monitoring program in operation during detection, characterization, and remediation of a release from a waste management unit. It is encouraging to note that all California landfills must have a vadose zone monitoring strategy to help prevent these solid waste sites from evolving into Superfund sites. Recognizing that the nation has thousands of solid waste sites, the EPA is developing a vadose zone closure guidance document for solid waste disposal sites in cooperation with the Vadose Zone Monitoring Laboratory at the University of California at Santa Barbara.

VADOSE ZONE MONITORING PROBLEMS

While the concept of vadose zone monitoring as a ground-water contamination prevention strategy began developing about 20 years ago, the theoretical basis was pioneered earlier by agriculturalists such as Buckingham, Gardner, and Richards (D.R. Nielsen, 1991). Before that time the vadose zone was considered the subsurface hydrologic “no man’s land” (Meinzer, 1942). Flanked above the soil zone studied intensively by agriculturalists, and below by aquifers investigated by hydrogeologists, this intermediate zone suffered for many years from a lack of interest by researchers who considered it too unimportant or too complicated for study.

As interest grows, environmental scientists from numerous disciplines are challenged by the intricacies of vadose zone phenomena. Unfortunately, many of these well-qualified professionals simply did not receive the specific training necessary to deal with counterintuitive subsurface flow regimes encountered in the vadose zone. Thus, when confronted with a subsurface monitoring problem, investigators have typically utilized what they were taught in school or on the job: traditional groundwater monitoring.

Lack of vadose zone education is exemplified by a common erroneous criticism of vadose zone monitoring when attempting to collect pore-liquid samples using vacuum lysimeters: “the samplers don’t work.” Inability to collect a sample does not typically indicate a failure of the instruments to operate properly. The majority of so-called failures are actually a failure to understand the principles governing unsaturated flow in the vadose zone. For example, no vacuum lysimeter can extract a pore-liquid sample from a soil with a matric potential greater than one atmosphere. From a practical standpoint, it is not possible to obtain a pore-liquid sample from most soils at matric potentials above 60 centibars. When the soil is so dry and the unsaturated hydraulic conductivity so low that pore-liquid samples cannot be collected, aqueous liquid migration is very close to nil. For this reason, “don’t work” can often be interpreted as “don’t worry.” Suspect lysimeters can be field tested in situ to determine if they have failed to function properly.

Inherited from the early disinterest in the vadose zone is a regulatory bias for ground-water monitoring, the currently preferred approach to “protecting” aquifers. This regulatory incentive to not detect contamination before it reaches an aquifer has an unfortunate ripple effect in the environmental monitoring instrumentation industry. Manufacturers and suppliers of monitoring equipment respond to market demand and the market is driven by the regulatory environment. Because the demand for vadose zone monitoring equipment remains relatively low, the economic incentive to research, develop, and bring new products to the market also remains low. Thus, the innovative new instrumentation required to develop tomorrow’s sophisticated vadose zone monitoring networks is not being brought to the market-place fast enough. For example, there is a need to reduce monitoring network space and time gaps to detect small or short-lived contaminant transport events. Currently available direct pore-liquid samplers have the drawback of a limited radius of sample collection and require a sampler spacing so close as not to be economically feasible for compl...