groundwater quality management

Groundwater Quality Management

GROUNDWATER QUALITY MANAGEMENT Bakenaz A. Zeidan Prof. of Water Resources, Head of Irrigation and Hydraulics Engineering Department, Faculty of Engineering, Tanta University, Egypt. [email protected]

Introduction In the past, water level was the main quantitative factor that determined the level of groundwater exploitation. Quality aspects decide whether groundwater is suitable for a specific use or not. Groundwater pollution occurs when pollutants are released to the ground and make their way down into groundwater. It can also occur naturally due to the presence of minor and unwanted constituents, contaminants or impurity in the groundwater, in which case it is more likely referred to as contamination rather than pollution. Pollution can occur from onsite sanitation systems, landfills, effluent from wastewater treatment plants, leaking sewers, petrol stations or from over application of fertilizers in agriculture. Pollution (or contamination) can also occur from naturally occurring contaminants, such as arsenic or fluoride. Using polluted groundwater is a hazard to public health through poisoning or the spread of disease. Different mechanisms have an influence on the transport of pollutants, for example diffusion, adsorption, precipitation, and decay in the groundwater. Movement of water and dispersion within the aquifer spreads the pollutant over a wider area. The advancing boundary, often called a plume edge, can intersect with groundwater wells such as seeps and spring, making the water supplies unsafe for humans and wildlife. Analysis of groundwater pollution may focus on soil characteristics and site geology, hydrogeology, hydrology, as well as the nature of the contaminants. The interaction of groundwater contamination with surface waters is analyzed by use of hydrology transport models.

Aspects of Groundwater Degradation The movement of contaminants through the subsurface is complex and is difficult to predict. Different types of contaminants react differently with soils, sediments, and other geologic materials and commonly travel along different flow paths and at different velocities. Groundwater

degradation occurs where there is excessive exploitation, for example where groundwater levels fall too fast or to unacceptable levels. This not only reduces available water resources and borehole yields but can result in other serious and potentially costly side effects including saline intrusion and subsidence; inappropriate at the land surface, including disposal of waste and spillage of chemicals, which contaminate the underlying aquifer. The nature of the aquifer will also influences the scale of the contamination problem. Thus, in a highly fractured aquifer where groundwater flow is easy and relatively rapid, contamination may become more widely dispersed in a given time than where flow is intergranular. Important issues when considering degradation are the use of water include Bakenaz A. Zeidan

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the availability of alternative sources and the scale of impact on different users. Degradation of groundwater often affects the poorest the most, as they are least the able to afford alternative water supplies or to cope with changes in livelihood that deterioration may force upon them [1,2]. Sources of Groundwater Contamination One of the challenges for hydrogeologists is to obtain meaningful chemical data from water samples collected from observation wells and monitoring wells to map the distribution of specific contaminants and to use as targets for any models that may be constructed to predict forward or backward in time. Contaminants found in groundwater cover a broad range of physical, inorganic chemical, organic chemical, bacteriological, and radioactive charcateristics as shown in Table 1.

Nitrate Nitrate can enter the groundwater via excessive use of fertilizers, including manure. High application rates of nitrogen-containing fertilizers combined with the high water-solubility of nitrate leads to increased runoff into surface water as well as leaching into groundwater causing groundwater pollution. The excessive use of nitrogen-containing fertilizers is particularly damaging. The nutrients, especially nitrates, in fertilizers can cause problems for natural habitats and for human health if they are washed off soil into watercourses or leached through soil into groundwater [3] as shown in Figure 1.

Fig. 1 Migration of Groundwater Contamination Source: http://water.usgs.gov/edu/pesticidesgw.html

Pathogens Waterborne diseases can be spread via a groundwater well which is contaminated with fecal pathogens from pit latrines. Pathogens contained in feces can lead to pollution when they are given the opportunity to reach the groundwater, making it unsafe for drinking. Groundwater that is contaminated with pathogens can lead to fatal fecal-oral transmission of diseases (e.g. cholera, diarrhea). Pit latrines can cause significant public health risks via contaminated groundwater impounds. Volatile organic compounds (VOCs) are a dangerous contaminant of groundwater. They are generally introduced to the environment through careless industrial practices [4].

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Arsenic contamination of groundwater This pollution occurs because aquifer sediments contain organic matter that generate anaerobic conditions in the aquifer. These conditions result in the microbial dissolution of iron oxides in the sediment and, thus, the release of the arsenic, normally strongly bound to iron oxides, into the water. As a consequence, arsenic-rich groundwater is often iron-rich, although secondary processes often obscure the association of dissolved arsenic and dissolved iron [5, 6].

Fluoride In areas that have naturally occurring high levels of fluoride in groundwater which is used for drinking water, both dental and skeletal fluorosis can be prevalent and severe [7].

. Fig. 2 Groundwater Contamination [5]

Landfill Leachate Leachate from sanitary landfills can lead to groundwater pollution which infiltrate into the water supply and evaporate in basements to further contaminate the air.

On-site sanitation systems In traditional rural housing compounds, where shallow water supply wells are in close proximity to the pit latrine, there is often contamination of the groundwater. Groundwater pollution with pathogens and nitrate can also occur from the liquids infiltrating into the ground from on-site sanitation systems such as pit latrines and septic tanks, depending on the population density and the hydrogeological conditions. This is a problem if a nearby water well is used to supply groundwater for drinking water purposes. During the passage in the soil, pathogens can die off or be adsorbed significantly, mostly depending on the travel time between the pit and the well. Most, but not all pathogens die within 50 days of travel through the subsurface. The degree of pathogen removal strongly varies with soil type, aquifer type, distance and other environmental factors [8], as given by Figures 2 and 3. Bakenaz A. Zeidan

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Sewage treatment plants The treated effluent from sewage treatment plants may also reach the aquifer if the effluent is infiltrated or discharged to local surface water bodies. Therefore, those substances that are not removed in conventional sewage treatment plants may reach the groundwater as well [10].

Hydraulic Fracturing The Environmental Protection Agency (EPA), along with many other researchers, have been delegated to study the relationship between hydraulic fracturing and drinking water resources. An evaluation of higher Helium and other noble gas concentrations along with the rise of hydrocarbon levels supports the distinction between hydraulic fracturing fugitive gas and naturally occurring "background" hydrocarbon content. This contamination is speculated to be the result of leaky, failing, or improperly installed gas well casings [11,12]. Table 1 Groundwater pollutants and their sources [9] Category

Contaminant Source

Agriculture

Animal burial areas - Animal feedlots - Fertilizer storage/useIrrigation sites- Manure spreading areas/pots- Pesticide storage/use Airports – Auto repair shops – Boat yards – Construction areas – car washes – dry cleaners- gas stations – jewelry/ metal plating – Laundromats – medical institutions- paint shops – photography establishments – railroad truck and yards – research laboratories – scrap and junk yards – storage tanks Asphalts plants – chemical manufactures/ storage – electronic manufacture – electroplaters – foundries/metal fabricators – machine/metal working shops – mining and mine drainage – petroleum production/storage – pipelines – sludge sites – storage tanks – toxic and hazardous spills – wells – wood preserving facilities Fuel oil – furniture stripping/refinishing – household hazardous products – household lawns – septic systems – sewer lines – swimming pools Hazardous waste landfills – municipal incinerators – municipal landfills – open burning sites – recycling/ reduction facilities – road deicing operations –road maintenance depots – storm water drains/basins – transfer stations

Commercial

Industrial

Residential

Other

Others Further causes of groundwater pollution are excessive application of fertilizer or pesticides, chemical spills from commercial or industrial operations, chemical spills occurring during transport (e.g. spillage of diesel fuels), illegal waste dumping, infiltration from urban runoff or mining operations, road salts, de-icing chemicals from airports and even atmospheric Bakenaz A. Zeidan

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contaminants since groundwater is part of the hydrologic cycle. Over application of animal manure may also result in groundwater pollution with pharmaceutical residues. Organic pollutants can also be found in groundwater, such as insecticides and herbicides, a range of organ halides and other chemical compounds, petroleum hydrocarbons, various chemical compounds found in personal hygiene and cosmetic products, drug pollution involving pharmaceutical drugs and their metabolites. Inorganic pollutants might include ammonia, nitrate, phosphate, heavy metals or radionuclides, [13], as shown in Figure 2.

Migration of Groundwater Contaminants Liquids spilled onto surface soils can migrate downward or can evaporate, which limits their potential for reaching the water table. Once below the water table, contaminants are also subject to dispersion (mechanical mixing with uncontaminated water) and diffusion (dilution by concentration gradients). The passage of water through the subsurface can provide a reliable natural barrier to contamination but it only works under favorable conditions. The stratigraphy of the area plays an important role in the transport of pollutants. An area can have layers of sandy soil, fractured bedrock, clay, or hardpan. Areas of karst topography on limestone bedrock are sometimes vulnerable to surface pollution from groundwater. Earthquake faults can also be entry routes for downward contaminant entry. Water table conditions are of great importance for drinking water supplies, agricultural irrigation, waste disposal (including nuclear waste), wildlife habitat, and other ecological issues.

Fig. 3 Groundwater Contamination from Septic Tanks Source: https://www.ec.gc.ca/eau-water/default.asp?lang=En&n=6A7FB7B2-1

Interactions with surface water Surface water seeps through the soil and becomes groundwater. Conversely, groundwater can also feed surface water sources. Interactions between groundwater and surface water are complex. Consequently, groundwater pollution, sometimes referred to as groundwater contamination, is not as easily classified as surface water pollution. By its very nature, groundwater aquifers are susceptible to contamination from sources that may not directly affect surface water bodies, and the distinction of point vs. non-point source may be irrelevant [14].

Seawater Intrusion in Coastal Aquifers Bakenaz A. Zeidan

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Seawater intrusion occurs near a coastline when a freshwater aquifer is depleted faster than it can be recharged. Seawater generally intrudes upward and landward into an aquifer and around a well, though it can occur “passively” with any general lowering of the water table near a coastline. The exploitation of coastal aquifers always produces a lowering of the water table level. When the extracted volumes are greater than the recharge, even on a local basis, a salinization process begins in the aquifer as the seawater flows upon the land. In the light of this, the management of coastal aquifers is conditioned by the need to determine the maximum permissible penetration limit for each particular aquifer [15- 18], as shown in Figure 4.

Fig. 4 Groundwater flow in coastal aquifers Source: http://www.atsdr.cdc.gov/sites/oakridge/contaminated_groundwater.html

Advection–Dispersion Solute Transport Most contaminants are introduced to the subsurface by percolation through the soil. The interactions between a soil and a contaminant are important for assessing the fate and transport of the contaminant in the groundwater flow system. Contaminants that are highly soluble, such as salts (e.g. sodium chloride, NaCl) move readily from surface soils to saturated materials below the water table. This often occurs during and after rainfall events. Those contaminants that are not highly soluble may have considerably longer residence times in the soil zone. Some contaminants adsorb readily onto soil particles and slowly dissolve during precipitation events, resulting in dissolve fraction concentrations of contaminants migrating to groundwater. This mode of transport is common for trichloroethylene. The most common mode of contaminant migration in the subsurface is advective flow with groundwater. Advective flow velocities are based on the average (bulk) properties of the aquifer materials and the average hydraulic gradient causing flow. This simple approach does not take into account dispersion, diffusion or adsorption of, contaminants, which can increase or decrease the rate of groundwater flow calculated by advection. Contaminant movement is also controlled by the process of mechanical dispersion. The amount of spreading is related to the dispersivity of the rock or sediment, the advective velocity of groundwater flow, and the molecular diffusion of the contaminant in the water in the pore space. The amount of diffusion is a function of the concentration gradient and the porosity of the materials [19-21].

Remediation of Contaminated Groundwater Groundwater pollution is much more difficult to abate than surface pollution because groundwater can move great distances through unseen aquifers. Non-porous aquifers such as clays partially Bakenaz A. Zeidan

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purify water of bacteria by simple filtration (adsorption and absorption), dilution, and, in some cases, chemical reactions and biological activity. However, in some cases, the pollutants merely transform to soil contaminants. Groundwater that moves through open fractures and caverns is not filtered and can be transported as easily as surface water. Remediation schemes can be categorized into three types; physical remediation, chemical remediation and biological remediation. Physical treatment may include filtering of water, etc. Chemical treatments include addition of chemicals that facilitate physical or biological treatments, or oxidize contaminants, etc. biological treatment (bioremediation) which utilizes complex biochemical pathways of various organisms to degrade or detoxify contaminants. The method of remediation depends on the chemical nature of the contaminant: pure-compound recovery is possible when groundwater is contaminated with a compound that is not water soluble. Pump and treat methods typically involve pumping contaminated groundwater out and treating it through air striping and chemical oxidation (using ozone, chlorine, etc.). In-situ remediation does not involve removal of the soil and groundwater. Organic contaminants like oil and gasoline are less soluble in water and can often be pumped off the surface of aquifers. Bioremediation is the use of living organisms like bacteria, plants, animals, or fungi to speed up the remediation process. Some of the biological treatment techniques include bio augmentation, bioventing, biosparging, bioslurping, and phytoremediation [22,23]. Pollutants and contaminants can be removed from groundwater by applying various techniques thereby making it safe for use. Most groundwater treatment techniques utilize a combination of technologies.

Fig. 5 Groundwater remediation scheme. Source: https://cluin.org/techfocus/default.focus/sec/Bioremediation/cat/Aerobic_Bioremediation_(Direct)/ Some chemical treatment techniques include ozone and oxygen gas injection, chemical precipitation, membrane separation, ion exchange, carbon absorption, aqueous chemical oxidation, and surfactant enhanced recovery. Some chemical techniques may be implemented using nanomaterial. Physical treatment techniques include, but are not limited to, pump and treat, air sparging, and dual phase extraction, as indicated by Figure 5.

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