5-1 5. Surface Water Quality Quality of water: Physical, chemical, and biological attributes that affect the sustainability of water for drinking, agriculture, recreation, and other uses. In this section, we will look at the pollution of surface water by suspended sediments and various municipal and industrial wastes. Effects of sediments Suspended load may be considered pollutant when it exceeds natural concentration. An increase in turbitity affects the biotic balance. For example, the increase in turbitity and the clogging of interstices in stream-bed gravel by silt has a detrimental effect on the survival of eggs and developing minnow of trout. Turbitity also reduces the depth to which sunlight penetrates and thus alters the rate of photosynthesis; e.g. algae growth. This may increase dissolved oxygen during hours of sunlight, but the decay of organic material consumes oxygen during hours of darkness. 5-2 Chemical characteristics of natural water Chemical characteristics : determined by the kinds of dissolved matters and their concentrations. Example: “clean” rain water - free from metals (Na+, Ca2+ etc.) and organic matter. - significant dissolved gases, in particular O2 and CO2, which play major roles in weathering processes. Concentration mg/L and ppm (parts per million) 1ppm = 1g/106g = 10-3g/103g = 1mg/L mol/L The mol is a unit to express chemical reactions. 1 mol of Na and 1 mol of Cl form 1 mol of NaCl. We could have said 22.99 g of Na and 35.45 g of Cl, but this is not so convenient. Notation: [Cl-] indicates the concentration of Cl in mol/L. pH pH = -log10[H+] H2O ⇔ H+ + OH[H+][OH-] = 10-14 In neutral water, [H+] = [OH-] = 10-7. i.e. pH = 5-3 pH of rain water Dissolved CO2 makes “pure” rain water acidic. H2O + CO2 ⇔ H+ + HCO3- (bicarbonate) Rain water is acidic (pH ≅ 5.6) without air pollution. With the addition of H2SO4 and HNO3 from burning fossil fuels, it becomes the “acid rain” (pH < 5.6). Chemical weathering H+ attacks rock-forming minerals. H+ + CaCO3(calcite) ⇔ Ca2+ + HCO32H+ + 2KAlSi3O8(feldspar) + 9H2O ⇔ Al2Si2O5(OH)4(kaolinite) + 4H4SiO4 + 2K+ In each case, H+ is neutralized by the mineral. Dissolution of calcite is fast. Calcite-rich soils offer a large buffering capacity. Dissolution of feldspar and other silicate minerals is very slow, not effective as a buffer. The prairie soil contains a large amount of calcite derived from limestone. Upon weathering, soil water gains the elements necessary for plant growth. Excessively high concentration, however, is harmful to plants. → soil salinity 5-4 Major ions Most natural water is characterized by: Ca2+, Mg2+, Na+, K+, Cl-, SO42-, and HCO3-. Note that pH of natural water is controlled by H2O + CO2 ⇔ H+ + HCO3Minor species Metals: Al, Si, Fe, Zn, etc. occurring in various forms Nutrients: NO3-, PO43-, and dissolved organic matter At high concentration, some species cause environmental problems. Solubility The maximum amount that can dissolve in water. When water becomes concentrated during evaporation, the concentration of some ions may become too high and the minerals start precipitating. Different minerals have different solubility. e.g. NaCl has high, and CaCO3 has low solubility. In general, the solubility of salts increases in this order, Ca < Mg < Na < K CO3 < SO4 < Cl CaCO3 is the first to precipitate, and KCl is the last. 5-5 Effects of pH on metal solubility Many metal species forms hydroxy complexes. Al3+ + H2O ⇔ AlOH2+ + H+ AlOH2+ + H2O ⇔ Al(OH)2+ + H+ This process is called hydrolysis. In low-pH water the abundance of the H+ ion prevents hydrolysis so that dissolved Al exists primarily as free Al3+ ions. Under neutral pH, a substantial portion of Al exists as Al(OH)3, which has a low solubility. Therefore, the total solubility of Al is the lowest in neutral water and higher in acidic and basic water. In this graph, the vertical axis is essentially the concentration of each species. Drever, J.I, 1988. The geochemistry of natural waters. Fig. 6-3. Fe3+ have similar tendency. Precipitation of hydroxides dictates the behavior of Al and ferric Fe in natural water. In general, solubility of metals drastically increase in lowpH water. 5-6 Cation exchange The surface of clay minerals have negative charge. Cations (Ca2+, Na+, etc.) are attached to the clay mineral surface by electrostatic force. Any given soil has fixed amount of Na+ Ca2+ clay, and hence fixed number of - - - - clay mineral negatively charged “sites”. All sites have to be occupied by cations. A site can exchange one cation with another. For example, a site can release 2Na+ and take Ca2+ from water in exchange. The negatively charged sites are called “exchange sites” and the number of exchange sites per unit mass of dry soil is called cation exchange capacity. Higher [Na+]/[Ca2+] in water results in higher Na+/Ca2+ on clay surface. When exchange sites are dominated by Na+, soils becomes unsuitable for growing crops. - poor drainage and aeration. Adsorption of metals and anions Some metals are adsorbed onto soil particles by the above mechanism of cation exchange, but others are adsorbed by more complicated mechanism. Also, negatively charged ions (anions) such as SO42-, NO3-, and PO43- are adsorbed by complex mechanisms specific to each ion. In general, PO43has much higher tendency to be adsorbed than NO3-. This dictates the behavior of the two species in the environment. 5-7 Oxidation Decay of organic materials CH2O + O2 → CO2 + H2O Pyrite oxidation and acid generation FeS2 + 3.75O2 + 3.5H2O → Fe(OH)3 + 4H+ +2SO42In these reactions, electrons are removed from carbon and sulfur atoms and added to oxygen atom. Oxygen is called electron acceptor, and carbon and sulfur are called electron donors. Each reaction release a certain amount of energy, which may be utilized by bacterial communities. In well aerated soil O2 is continuously replenished; this is called aerobic condition. In poorly aerated environments O2 is quickly used up by reactions. This is commonly called anaerobic condition. Under anaerobic condition, species other than O2 are used as electron acceptors; (denitrification) NO3- → N2 SO42- → H2S (sulfate reduction) CO2 → CH4 (methanogenesis) The condition becomes more strongly anaerobic, or reducing, as NO3- and SO42- are used up. In general, more energy is released by reactions involving O2 than NO3-. In other words bacterial communities consume O2 first, then NO3-, and then SO42-. Therefore, denitrification does not occur under aerobic condition. 5-8 Salinity of water Total dissolved solids (TDS), expressed as mg/L. Sea water : 35,000 mg/L River water : 100 - 500 mg/L TDS higher than a few thousands mg/L has adverse effects on crops. TDS is related to the electrical conductivity (EC). In a rough sense, TDS (mg/L) ≅ 0.7 EC (µS/cm) Evaporative enrichment Suppose 1 L of water in a pan, having TDS of 100 mg/L. Evaporate 900 mL, TDS = Composition also changes. Start with (Ca, Mg, Na) - (HCO3, SO4). As CaCO3 precipitates, water becomes more enriched in Na, Mg, and SO4. Origin of salts in the prairies Glacial tills contain limestone and shale, which are high in CaCO3 and FeS2, respectively. FeS2 + 3.75O2 + 3.5H2O → Fe(OH)3 + 4H+ +2SO422H+ + CaCO3 + SO42- → Ca2+ + SO42- + CO2 + H2O The “unoxidized” dark gray glacial tills have significant FeS2. The “oxidized” olive brown tills have significant Fe(OH)3. Soils derived from the oxidized tills have high [HCO3-] and [SO42-], so as groundwater in the prairies. 5-9 Major ions affected by human activity Return flow from irrigated fields has raised concentration of major ions due to evaporative enrichment and the dissolution of minerals and organic matter. If the stream water is used several times over for irrigation, the salinity of water in rivers or canals increases downstream. Water may become unsuitable for irrigation. Change in salt content of the Sevier River, Utah, as a result of repeated diversion for irrigation. (D&L, Fig. 20-4) Urban and industrial use also increases salt concentration. Treated municipal sewage adds to the receiving water about 35 kg of inorganic salts per person per year. The water used in industrial plants picks up a large quantities of solutes. Besides the direct impact of municipal and industrial waste disposal, the landuse may impact the salt concentration in rivers by drastically altering the hydrologic cycle. 5-10 Trace metals Heavy metals such as Fe, Zn, Pb, and Cu have low solubility in neutral-pH water and are often mobilized by forming soluble complex with organic molecules or by becoming adsorbed to clay particles. At high concentration, they can be toxic and can cause major disruption of aquatic ecosystems. In Canada and US, waste water from industrial plants are treated to reduce the concentration of heavy metals. However, groundwater originating from acid mine tailings, when it discharges into stream, may lower the pH of water and thereby dissolve metals. Note that fish are not killed by low pH itself, but rather by the increased concentration of metals. In particular, moderately high concentration of Al can cause a serious damage to gills of some species. The acidity comes from the oxidation of metal sulfides, which is almost unavoidable in many operating and decommissioned mines. The emission of SO2 and NOx from industrialized area causes acid rain. The catchments surrounding prestine lakes on the Canadian Shield has very thin soil cover, which offers little buffering capacity against acidification of lakes. Suggested reading: Schindler, D.W., 1988. Effects of acid rain on freshwater ecosystems. Science, 239: 149-157. 5-11 Biodegradable wastes and the oxygen balance The major consumption of dissolved O2 in streams occurs through the aerobic chemical and microbial breakdown of long-chained organic molecules into simpler, stable endproducts. carbohydrate → CO2 + H2O proteins → amino acids → ammonia → nitrite → nitrate → sulfate and phosphate If O2 is exhausted, aerobic decomposition ceases and further breakdown must be accomplished by anaerobic bacteria. A moderately high dissolved O2 is necessary for the maintenance of healthy aquatic ecosystems. When a large amount of industrial or municipal waste enters the stream, the breakdown of the waste may depletes O2 in the stream. The pollution strength of a waste is measured by Biochemical Oxygen Demand (BOD), which is the amount of O2 consumed by living organisms (mostly bacteria) while utilizing the organic matter in the waste. The biochemical consumption of O2 is a complex process that may take weeks and is dependent on many environmental factors (see next page). It is usually measured under arbitrary conditions in the laboratory; 5-day digestion at 20 ºC. Chemical Oxygen Demand (COD) can be used to obtain a rough estimate of BOD because it can be measured in a few hours. 5-12 D&L, Fig. 20-7 D&L, Table 20-4 Table 20-4 lists 5-day BOD of various wastes. For example, 1 L of untreated domestic sewage demands up to 400 mg of dissolved O2. D&L, Table 20-6 Table 20-6 lists the solubility of O2 in water. At 20 ºC, it takes 44 L of water to accommodate 400 mg of O2. 5-13 When a waste enters a stream, it becomes completely mixed with stream water after a short distance. The assimilation of oxidizable materials begin to consume O2 and increase O2 deficit. O2 is replenished by reaeration from the atmosphere, the rate of which depends mainly on the O2 deficit, the width of the stream, turbulence, and water temperature. The balance between O2 consumption and replenishment leads to a profile of net O2 deficit, which shows a characteristic dissolved-oxygen sag. D&L, Fig. 20-9 5-14 The profile of O2 deficit is estimated by the Streeter-Phelps equation; kL − kt − rt − rt D= where D: Di: k : L: r : t : r−k (e −e ) + Di e O2 deficit of the stream [mg/L] initial O2 deficit of the waste mixture [mg/L] rate constant of O2 consumption [day-1] initial BOD of the waste mixture [mg/L] reaeration rate constant [day-1] time from initial mixing [day] The word “initial” indicates the time when the waste is completely mixed with stream water. The initial waste mixture may have significant O2 deficit. The rate of O2 consumption per unit BOD is dependent on the type of waste. Many agricultural processes release wastes that can be rapidly oxidized. In contrast, pulp wastes are assimilated slowly. Therefore, the rate constant k depends on the type of materials in the waste. The constant r represents how fast O2 is being replenished. Both k and r are dependent on water temperature. The S-P equation is very simplistic, but it illustrates how the O2 sag depends on several important factors. Environmental planners need to be familiar with this concept so that they can effectively interact with sanitary engineers. For example, what happens if r is drastically reduced by ice covering a major portion of the river? 5-15 Variation of the dissolved O2 sag. Q is the stream discharge. The lower Q results in the higher initial BOD of the waste mixture. (D&L, Fig. 20-12) 5-16 Strategies for reducing dissolved O2 problems The BOD in wastes can be reduced by a set of treatment processes. Primary treatment involves the settling of suspended mineral and organic matters, which removes 4090 % of the suspended solids and 25-85 % of the 5-day BOD. Secondary treatment involves biological processing. In a trickling filter, wastewaters are sprayed onto columns of crushed stone and flow in thin films. Organisms grazing on the surface of stone break down the dissolved organic materials. In a activated sludge process, flocs of bacteria are stirred in the wastewater. These processes remove 5095 % of the BOD. Tertiary treatment includes such practices as using a strong oxidizing agent like ozone to remove BOD and the addition of alum to precipitate phosphates which are not removed by other treatment processes. Alternatively, wastes can be processed in stabilization ponds in which aerobic microbial organisms digest the organic substances, but the pond is also colonized by aquatic plants, which release oxygen during photosynthesis. This method of treatment is gaining a large popularity in recent years and more and more artificial wetlands are constructed for this purpose. More advanced reference: Linsley et al., 1992. Water-resources engineering. 4th ed., Chapter 19. 5-17 Plant nutrients and eutrophication The breakdown of organic compounds (see 10-16) removes BOD, but it generates nitrates and phosphates, which are the stable forms of nitrogen and phosphorous under the aerobic condition. The concentration of NO3 and PO4 increases immediately below the outfall of waste and decreases gradually due to dilution. Note that the majority of PO4 exists in the forms HPO42- and H2PO4- in neutral-pH water. PO43- + H2O ⇔ HPO42- + OHHPO42- + H2O ⇔ H2PO4- + OH- Diagram of typical changes in water chemistry (D&L, Fig. 20-17). N and P are among the essential nutrient elements for plants. Eutrophication occurs when the excess nutrients causes a significant increase in the rate of plant growth. In particular algae can grow prolifically (algal bloom) with the excess nutrients to cause a number of problems. 5-18 Algal blooms are unsightly; they may increase the cost of water treatment; and upon their death and decay they impose an O2 demand on the water, which may lead to fish kills. Plant growth is usually limited by the availability of one or a few of the essential nutrients, for example P, N, and C. The nutrient that most frequently limits algal production is P. Note that N and C are contained in air. Therefore, by the diffusive exchange, the algae can take those elements from the air. P is only available from the transport of dissolved PO4 by rivers or groundwater. PO4 easily becomes immobilized in the soil by forming insoluble calcium phosphate minerals. Natural waters from undisturbed forest has dissolved PO4 concentration of 0.0050.05 mg/L. Usually 0.08-0.1 mg/L is necessary to trigger blooms. The bulk of P transport in streams occur as organicP and PO4 attached to suspended sediments. These “immobile” form of P may become available to plants later. In agricultural lands, P is applied as fertilizers. The plant uses only about 15 % of applied P, and the rest is fixed in the soil. Therefore, P accumulates in the soil every year. Soil erosion can release a large amount of P in streams and lakes. Animal wastes from feedlots contains large amount of P. Domestic sewage is an important contributor of P, because treatment processes remove only 20-30 % of P. 5-19 Excess N originates from the same source as P. The most stable form NO3- is highly mobile, in contrast to immobile PO4. Agricultural regions, heavily fertilized with N, may yield runoff containing NO3- exceeding 1 mg/L. Groundwater beneath fertilized field can have concentration exceeding 100 mg/L. High concentration (> 45 mg/L) of NO3- in drinking water can cause methemoglobinemea (blue-baby syndrome) among infants. Domestic sewage averages about 20 mg/L of N, because treatment processes cannot remove the majority of N. Point source: industrial and domestic sewage, feedlots, etc. Non-point source: agricultural runoff, etc. Point sources are easier to control than non-point sources. For point sources, the nutrients can be removed by passing the effluent through a treatment pond (artificial wetland) where aquatic plants and microbial organisms consumes the nutrients. For non-point sources, riparian buffer zone can be placed along the river bank or lake shore, where the lush vegetation captures suspended soil particles in runoff water and removes the nutrients. However, a narrow buffer zone may not be effective in removing the nutrients in groundwater, which discharges into rivers and lakes.