Environment Canterbury has a legal responsibility to monitor the quality of groundwater and surface waters in the region. There are many reasons for doing this:
This is measured either as the concentration of oxygen dissolved in the water (expressed as grams of oxygen per cubic metre of water), or as the proportion of oxygen actually present relative to the theoretical oxygen-holding capacity of the water (expressed as "percentage saturation"). The latter measure is sometimes preferred because the ability of water to hold oxygen varies with temperature.
Dissolved oxygen is a basic requirement for a healthy aquatic ecosystem. Most desirable fish species (such as trout and salmon) suffer if dissolved oxygen concentrations fall below 3 to 4 g/m3. Larvae and juvenile fish are more sensitive, requiring even higher concentrations of dissolved oxygen. Prolonged exposure to low dissolved oxygen conditions can suffocate adult fish or reduce their reproductive survival by suffocating sensitive eggs and larvae. Fish can starve when aquatic insect larvae and other prey die in response to the altered conditions. Low dissolved oxygen concentrations also favour anaerobic (without oxygen) bacterial activity that produces noxious gases or foul odours often associated with polluted water bodies.
Introducing large quantities of biodegradable organic materials, such as sewage or food processing wastes, into surface waters can rapidly consume available oxygen.
Bacteria use oxygen to break apart (or decompose) organic materials. Pollution containing organic wastes provides a continuous supply of food for the bacteria, which accelerates bacterial activity and bacterial population growth. In polluted waters, bacterial consumption of oxygen can rapidly outpace oxygen replenishment from the atmosphere and photosynthesis performed by algae and aquatic plants. The result is a net decline in oxygen concentrations in the water.
Other factors, such as temperature and salinity influence the amount of oxygen dissolved in water. Prolonged hot weather will depress oxygen concentrations and may cause fish kills even in clean waters, because warm water cannot hold as much oxygen as cold water. Warm conditions further cause oxygen depletion by stimulating bacterial activity, which consumes oxygen.
Temperature is a fundamental factor in water quality, and the temperature exerts an enormous influence over aquatic organisms. If the overall temperature of an aquatic system is altered, a shift in community composition can be expected. Cold water fish such as trout and salmon are very sensitive to temperature change, and as temperatures increase above about 20oC suffer physiological stress. Fish are also potentially affected when swimming into localised areas of warm water. This impact is particularly relevant to braided rivers in Canterbury where water levels can drop rapidly in some channels. The size of the temperature change varies greatly from river to river and between rivers, depending on interactions between surface waters and groundwater inflows, as the later can have a substantial buffering effect on temperature. Higher temperatures can also compound low dissolved oxygen concentrations in lakes and closed river mouths, by encouraging the decomposition of organic material, and reducing the oxygen-carrying capacity of the water.
Many factors affect water temperature. These include large fluctuations in air temperature, changes in the shape of stream channel and lake margins, reductions in overhanging vegetation, cloudiness, and most importantly, reductions in water flow. Wastes discharged into water can also affect temperature, if the effluent processing or treatment temperature is substantially different to the background water temperature. In this regard, discharges of water used for cooling in industrial processes such as heat exchanges can be considerably warmer than the water into which they are discharged. This will depend on the time of the year that the discharge occurs and the natural temperature regime of the water body. In summer, such discharges may actually have a cooling effect.
Acidity and alkalinity, the concentration of hydrogen and hydroxyl ions in water, drive many chemical reactions in living organisms. The standard measure of acidity and alkalinity is pH, and a pH value of 7 represents a neutral condition. A low pH value indicates acidic conditions; a high pH indicates alkaline conditions.
Most waters in Canterbury have a fairly neutral pH. There are few, if any, surface waters where biological processes are adversely affected by pH changes, with the possible exception of some river mouths that are cut off from the sea in summer, and small streams with low summer flows and extensive plant growths.
The pH of most groundwater is in the range recommended for drinking purposes of 6.5-8.5. However, shallow unconfined groundwater (usually from wells less than 30 m deep) can be slightly acidic - down to 6.0. This is because rainwater (itself slightly acidic) carries carbon dioxide (produced by plant roots and micro-organisms) into the underlying groundwater where carbonic acid is produced. Low pH will not affect your health on its own (e.g., lemonade and other carbonated drinks have a pH of about 3), but slightly acidic water is corrosive and can dissolve metals, especially copper, from pipes and pumps, into the water. Increasingly, copper piping is being replaced by PVC, which is not affected by the water pH. However, the economic life of hot water cylinders may be shortened owing to the water acidity.
This measures how electrically conductive the water is. Because conductivity increases with the number of ions (electrically charged particles) in the water, it indicates the presence of dissolved substances. Such substances may be naturally occurring minerals (see cations, and anions), or could be contaminants that are in the water as a result of human activities. Conductivity is usually measured in the field using a hand-held meter. A common unit of measurement is millisiemens per square metre, or mS/m3.
The nutrients most often responsible for water quality degradation are nitrogen and phosphorus. Because these are found in the environment in a number of forms, water quality scientists measure them in different ways. For example, nitrogen present in water may be bound up in plant or animal tissue, in which case it is referred to as "organic" nitrogen. Such nitrogen eventually breaks down into "inorganic" forms; nitrate (NO3), nitrite (NO2) or ammonia (NH3). The relative proportions of these different forms provide clues as to the possible sources of nutrient-rich contaminants, or the time since their discharge to the water body. Sources of nutrients include plant fertilisers, sewage effluents, animal and food-processing wastes, and urban stormwater.
Nutrients are essential building blocks for healthy aquatic communities, but excess nutrients (especially nitrogen and phosphorus compounds) over-stimulate the growth of aquatic weeds and algae. Aquatic weeds and algae out-compete the native submerged aquatic vegetation and can smother the habitat used by the aquatic fauna.. Decomposition of excess weeds and algae can lead to oxygen depletion. Surface waters that have high concentrations of nutrients are referred to as "eutrophic". The adverse effects of high nutrient concentrations are particularly noticeable in lakes, where the nutrients are recycled through the same water, and tend to gradually accumulate.
Sources of nitrate are the same as those for ammonia. This is because an important mechanism for the formation of nitrate in soil and water is the breakdown (or "mineralisation") of ammonia. Nitrate occurs in groundwater sometimes from the decomposition of crop residues that leach down from the soil. This occurs when more nitrate is present than is required as a nutrient for plant growth. An important source of nitrate in New Zealand agricultural systems is derived from nitrogen fixing plants such as clover that capture nitrogen from the air. Also important as a source of nitrate in groundwater is urine from grazing stock.
Nitrate is measured as nitrate (NO3), or as nitrate in the form of nitrogen (NO3-N). The New Zealand drinking-water Standard (2000) maximum acceptable value (MAV) is 50 mg/l of nitrate. When the ion is reported as nitrate-nitrogen the equivalent MAV is 11.3 mg/l. These drinking water standards are based on the prevention of adverse health effects on pregnant women and bottle-fed infants up to 6 months old. Older humans can take in higher nitrogen concentrations without undue health effects. The standard for drinking water is exceeded in some areas of shallow unconfined groundwater in Canterbury where there is little dilution from rivers, and rainfall carries nitrogen from the soil into groundwater. Nitrogen content also increases in groundwater when the water table rises, and flushes nitrogen from the base of the soil profile.
Sediment consists of particles of all sizes, including fine clay particles, silt, sand, and gravel. In a water quality context the particles of greatest concern are the fine clays and silts. Sediment in the water column is usually referred to as suspended sediment, and measured as a concentration in g/m3.
When sediment settles out it can severely alter aquatic communities. Sediment may clog and damage fish gills, suffocate eggs and aquatic insect larvae on the bottom, and fill in the spaces between gravel where fish lay eggs. Suspended silt and sediment interfere with aquatic pant growth by reducing water clarity. Similarly they can impair recreational activities and aesthetic enjoyment by altering the appearance of the water.
Sediment may also carry other pollutants into water bodies. Nutrients and toxic chemicals such as heavy metals may attach to sediment from where they are carried into surface waters. There, the pollutants may settle with the sediment or detach and become soluble in the water column.
Rain washes silt and other soil particles off all surfaces, but particularly those where the vegetative cover has been disturbed. Consequently, soil erosion, and activities such as earthworks, vegetation clearance, and cultivation can result in sediment movement into surface water, particularly after heavy rainfall. Stock trampling in the bed of a stream or trampling the margins and banks can release large amounts of sediment into the water.
A water quality measure that is related to suspended sediment is turbidity. This quantifies the degree to which light travelling through water is scattered by the suspended particles present. The greater the amount of suspended material, the greater the light scattering and the higher the turbidity. The light-scattering particles may be both organic (e.g., algae and other plant or animal debris) or inorganic (e.g., fine silts or clays).
High turbidity affects the aesthetic appeal of waters, and in the case of recreational areas may obscure hazards for swimmers and boaters. Its environmental effects are essentially the same as those for suspended sediment: reduction in light penetration reduces plant growth, which in turn reduces the food source for invertebrates and ultimately fish. If turbidity is largely caused by organic particles, their microbial breakdown can lead to oxygen depletion, the stimulation of algal growth, and the other adverse effects associated with nutrient enrichment.
Turbidity is measured in a special type of light meter, and is generally expressed in Nephelometric Turbidity Units (NTU). An NTU less than 25 is considered acceptable for aquatic life, but the appearance of the water is affected at much lower values than this.
Certain bacteria, viruses and protozoa cause human illnesses that range from gastro-intestinal disease to minor respiratory and skin diseases. These organisms may enter waters through a number of routes, including inadequately treated sewage, stormwater drains, septic tanks, runoff from pastoral farm land, animal processing plants and from wild life living in and around water bodies. Groundwater can become contaminated with micro-organisms being leached from the land into groundwater. Shallow, unconfined groundwater (less than 30 m deep) is vulnerable to microbial contamination from a number of sources including septic tank discharges, grazing stock, effluent discharges and offal pits. Contaminants can commonly enter the groundwater directly via poorly protected well heads.
Because it is impossible to test waters for every possible disease-causing organism, it is usual to measure indicator bacteria that are found in high numbers in the stomachs and intestines of warm-blooded animals and people. The most frequently used organisms for this purpose are faecal coliforms, or a species of these, Escherichia coli. For marine waters, a group of bacteria known as Enterococci is now commonly used. The presence of such bacteria indicates the possible presence of faecal material and, with it, the possibility that other, disease-causing organisms may be present. Indicator bacteria concentrations are used to determine if water quality is adequate for contact recreation, or as a source of drinking water.
Indicator bacteria are measured as a concentration, usually expressed as an estimate of the number of individual organisms per 100 ml of water. Water quality managers are interested in both the concentration of single samples, and in the "average" concentration of a series of samples taken over a period of time. For example, for water to be suitable for drinking, faecal coliforms should not be detected in any samples. For marine bathing waters, no samples should exceed an Enterococci concentration of 277 per 100 ml, and the median concentration should not exceed 35 per 100 ml.
Cations are a group of substances which, when dissolved in water have a positive electric charge. The main cations found in both surface and groundwaters are calcium (Ca), magnesium (Mg), iron (Fe) and manganese (Mn). Other cations include potassium (K) and sodium (Na). The presence of these ions is primarily a consequence of the geology of the area through which the water has flowed. For example, waters moving through limestone will contain high concentrations of calcium, magnesium and bicarbonate. These ions contribute to the hardness of water. Groundwater moving through dark coloured (mafic) volcanic rocks will generally contain lots of iron and manganese.
High iron and manganese concentrations can also occur in water from wells drawing from groundwater zones low in oxygen, particularly when these are associated with peaty deposits. Any iron or manganese present will be in solution in the ground, but will precipitate out of solution when oxygen is added as the groundwater is drawn up to the surface. Consequently pipework, pumps, trickle filter irrigation systems, and showerheads can clog up with iron oxide (red) or manganese oxide (black) deposits. Clothing being washed in this water will become stained, and the water will taste peculiar. These nuisance effects occur at concentrations below that which would cause a health problem. However, high concentrations of manganese can pose a health risk if used as a drinking-water. Pumping the well to remove any stagnant water in the casing before using the water helps to remove accumulated contaminants. Sometimes the iron and manganese can be removed from the water by filter systems or other treatments but filters need to be cleaned regularly. However, as long as the well draws water from the same place in the aquifer the problem will continue.
Hardness is a measure of how much calcium and magnesium is present in the water. Hard water makes it difficult to lather up soap and can cause scale to develop on the insides of pipework. Consequently very hard water cannot be used in commercial boilers. Soft water (i.e., water with very low calcium or magnesium concentrations) can corrode metals from pipework, because it tends to be slightly acidic (see pH).
Hardness, whether it is caused predominantly by calcium, magnesium, or both together, is usually reported as the equivalent total concentration of calcium carbonate (CaCO3). Waters with a CaCO3 equivalent concentration greater than 120 gm-3 are considered to be hard.
Anions are a group of substances which, when dissolved in water have a negative electric charge. The main anions of interest from a water quality perspective are sulphate (SO4), chloride (Cl), nitrate (NO3) and bicarbonate (HCO3). Anions may indicate the source of the water, or factors that influence its quality. For example, high concentrations of chloride in groundwater may indicate that saltwater is entering the system.
Toxic organic chemicals are synthetic compounds that contain carbon, such as polychlorinated biphenyls (PCBs), dioxins, and the pesticide DDT. These compounds often persist and accumulate in the environment because they do not readily break down in natural ecosystems. Thus, they are often referred to as persistent bio-accumulative toxins. Many of these compounds cause cancer in people and birth defects in other predators near the top of the food chain, such as birds and fish.
An important group of organic compounds for water quality in Canterbury, particularly as far as groundwater is concerned, are hydrocarbons. These chemicals are used in petroleum products, refrigerators, insecticides, solvents, propellants, and cleaners. They can contaminate water as a result of spillage or disposal. Because hydrocarbons are frequently stored in underground tanks, they pose a potential risk to groundwater in the event of tank rupture.
Pesticides are another group of chemicals that can contaminate water. A pesticide is any substance that is used to control a particular organism. Pesticides include fungicides, insecticides, herbicides and growth regulators. Generally pesticides are transported into surface waters via rainfall runoff from areas such as road surfaces or farmland that have been sprayed. Occasionally they may be found in streams as a result of over-spraying or spray drift, or when they have been applied directly to stream channels to control aquatic weeds. Most pesticides stick strongly to soil particles or break down quickly, so it is unusual to find them in groundwater. However, some pesticides are very mobile, and can leach through the soil into groundwater (e.g., the triazine herbicides: simazine, atrazine , terbuthylizine). These same triazine herbicides are also frequently found in streams and rivers in intensive agricultural areas, as well as some of the phenoxy-acidic herbicides such as MCPA.
Pesticides can also get into groundwater as a result of spillage near a well, or by back-syphoning when they are being prepared prior to application
High density metals such as lead, zinc, copper and chromium and metalloids such as arsenic occur naturally in the environment. However, human activities (such as industrial processes and mining) have altered the distribution of metals in the environment.
Heavy metals and metalloids can be associated with both surface and groundwater contamination. In surface waters metals are usually found in association with sediments, to which they attach readily. Consequently, sampling for metals in the water column is often of limited value. A more meaningful measure of environmental contamination is found by sampling sediments in the bed of streams or by analysing animal or plant tissue for heavy metals. While some animals exposed to contaminated sediments are able to regulate the amount of metals in their body tissue, animals higher up the food chain, such as fish are more likely to accumulate metals in increasing concentrations.
The tendency of metals to attach to fine-grained sediment or organic matter means that in groundwater they do not usually move very far from their source. An exception is when the water is very acid; in such conditions the metals are less likely to bind to particles, remaining mobile or where the groundwater is reduced (no oxygen content) which can increase the solubility of some compounds.
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