Erik Romijn and Alexander Zaporozec
7.4 Remediation of contaminated groundwater
7.4.4 Active in-situ remediation
With the limitations of pump-and-treat in mind, more appropriate in-situ technologies were developed (Zaporozec, 1998). Among these are soil vapour extraction, biodegradation, bioventing, air sparging, and electrochemical concentration. Soil vapour extraction is used to remove volatile organic compounds from the unsaturated zone by suction wells. It can be combined with bioventing, a delivery of oxygen to the soil. In-situ biodegradation uses microorganisms to convert contaminants to less harmful forms. This method has the added benefit that the contaminated soil and groundwater do not need to be disturbed. Biodegradation is enhanced by supplying electron acceptors (oxygen and nitrate) to stimulate bacterial growth and break down organic compounds.
Air sparging, a process similar to bioventing, is used to deliver air below the water table.
Again, these systems only work if the soil and the aquifer are sufficiently permeable and do not clog during oxidation, for example, with iron hydroxides. Further, the partitioning of the contaminant between soil particles, water, and air should be favourable, which means that the aquifer must have a low adsorption capacity for the contaminants.
In electrochemical concentration, contaminating ions can be transported by an electric field, which is induced by electrodes installed underground. By electrolysis, the cations (e.g. heavy metals) will move to the cathode, while their water mantles are carried with them; this is called electro-osmosis. The metal ions, collected at the cathode, have to be constantly removed by a circulating fluid in the cathode box. The same holds for the anode with respect to anions.
Acidification will enhance the performance. Iron objects underground disturb the electric field and lower the efficiency of this process.
7.5 Monitoring
Groundwater monitoring is one of the important methods supporting the strategy and policy of groundwater protection. Because no preventive control system or remedial technology is
Groundwater contamination inventory
100 percent effective or complete, groundwater quality must be monitored at key points. In general terms, monitoring is the continuous, standardised measurement and observation of the environment (UNESCO/WHO, 1978). In terms of groundwater protection, monitoring is an important device to detect groundwater contamination and to provide an advanced warning of contaminated groundwater approaching important sources of water supply. In addition, ground-water quality monitoring can help define the extent of groundground-water contamination and control the quality of drinking water.
According to Vrba and Sobíšek (1998), groundwater quality monitoring ‘has the following objectives:
•
to collect, process and analyse background data on water quality and quantity as a baseline for evaluating the current state and for anticipating the changes and trends of the hydrogeological system, and•
to provide information for the planning, management, and decision-making about groundwater resource development, protection, and conservation and for the imple-mentation of legislative and control measures and regulations.’The goals of any proposed groundwater monitoring program should be clearly stated and under-stood before decisions are made on the types and numbers of wells needed and their locations and depths, constituents of interest, and water sampling procedures. The placement and number of wells will depend on the results of contamination source inventory, complexity of hydrogeological setting, and degree of temporal and spatial detail needed to meet the goals of monitoring program (Barcelona et al., 1987).
Groundwater quality monitoring is a technically and financially demanding process. There-fore, the benefits and information derived from the monitoring should always be compared with the cost of obtaining this information (Vrba and Sobíšek, 1988).
Groundwater monitoring programs can operate at the international, national, provincial/
state/regional, or local (site-specific) level. The type and density of monitoring stations for these four categories and sampling frequency and requirements were described by Meybeck (1985) within the framework of the Global Environmental Monitoring System (GEMS), and are included in Table 7.4.
TABLE7.4 Categories of groundwater monitoring stations operating in groundwater quality monitoring programs (Source:Meybeck, 1985)
Category and importance Monitoring of monitoring station
Station Sampling Variables Characteristics of monitoring station program Baseline Trend Impact density frequency analysed
International D C LS VL L B + Baseline station: natural background
O - groundwater quality.
National D C LS L L B +
O - Trend station: trends in groundwater quality due to natural processes and
Provincial/ C D LS M M B + human impacts.
State O +
Local LS LS D H H O + Impact station: changes of
groundwater quality due to various human impacts.
Station significance: D - Dominant, C - Complementary, LS - Low significance.
Sampling frequency: H - High: more than 12 times a year, M - Medium: 2 to 12 times a year, L - Low: 1 to 4 times a year.
Variables analysed: B - Basic: physical, chemical, and biological variables included into
Each purpose for groundwater quality monitoring must satisfy somewhat different requirements, and may require different strategies for well location, design, and construction. Four basic types of monitoring in relation to groundwater protection can be distinguished (Barcelona et al., 1987):
1) Ambient trend monitoring: measurements of groundwater quality to establish an overall picture of temporal and spatial trends within a groundwater basin or region. Existing public and private wells are used to the maximum extent possible.
2) Source monitoring:measurements of effluent quality for contamination sources that may affect groundwater, usually done for regulatory purposes. Monitoring wells are located and designed to detect the movement of contaminants from a given source or activity (early warning or offensive detection monitoring).
3) Case preparation monitoring:carefully documented measurements within a given area to gather evidence for legal proceedings or enforcement actions of past, existing, or antic-ipated groundwater contamination situations. Requires a level of detail similar to source monitoring.
4) Research monitoring:investigations on groundwater quality and contamination occurrence and movement. Requires a sophisticated level of detail to expand the understanding of complex mechanisms of groundwater movement and solute transport.
In the design of groundwater quality monitoring program four basic factors must be considered (Vrba and Sobíšek, 1988):
1) monitoring objectives;
2) extent of the area to be monitored;
3) duration of monitoring;
4) potential impacts on the groundwater system and their effects.
Design of a groundwater quality monitoring program should be flexible and continuously adjusted in response to the movement of a contamination plume or to the effects of groundwater remediation efforts. The elements of a groundwater quality monitoring system are shown in Fig. 7.2.
Groundwater contamination inventory
FIGURE7.2 Simplified scheme of development of a monitoring system (Source:Vrba and Sobíšek, 1988)
For details on monitoring objectives and strategies, well design, and water quality sampling the reader is referred to the report on IHP-V Project 3.2 Monitoring strategies for detecting ground-water quality problems (Vrba, 2002).
Inventories of contamination sources widely vary in terms of their purpose and scope. To demonstrate this variety, a few examples from various regions of the world are included in this chapter. They may provide a better picture of why and how various nations approach the inventory.
The presented case studies describe the inventories of anthropogenic sources of contam-ination at a local level (case studies 8.4 and 8.6 from Nicaragua and the United States, respec-tively); at a regional level (case studies 8.2 Brasil and 8.5 South Africa); or at a national level (Italian case study 8.3). However, many inventories are carried out also for the purpose of determining the extent of natural contamination sources. A prime example is the case study from Bangladesh (8.1), which was set up to evaluate the causes and origin of poisoning of drinking-water supplies by naturally-occurring arsenic.
The goal of the local inventory of contamination sources in the Managua area, Nicaragua, was to characterise contamination sources and quantify the potential contamination load to the groundwater for groundwater protection purposes in an urban, suburban, and rural area with rapid, partly spontaneous urban growth. The local inventory of contamination sources in south-eastern Wisconsin, USA – a mix of urban, suburban, and rural land with local concentrations of industries – is an example of an inventory conducted with limited financial and manpower resources, which resulted in a qualitative assessment of the nature of predetermined sources and of the extent of potential contamination problems within the framework of a long-term, regional water quality plan.
The case study evaluating the contamination threat to aquifers in the state of São Paulo, Brasil, is an example of a regional inventory of contamination sources in an area of intense industrial and agricultural activity. In this case, screening of contamination sources was necessary to prioritise the activities according to their groundwater contamination risk. Disposal of waste is of great concern in South Africa. An example of the evaluation of the suitability of land for a regional waste disposal site in the Western Cape Province demonstrates the methodology for site selection. Even though this case study does not discuss contamination inventory per se, it outlines a process for identifying suitable areas for an important potentially contaminating activity.
Inventory of contamination sources at the national level is represented by the study of the contamination of drinking-water supplies in Italy and its causes. The inventory was conducted within the territory of individual municipalities, and the contamination sources were classified into seven categories: industrial, urban, agricultural, stock breeding, waste disposal, seawater intrusion, and naturally-occurring substances.