4. 1.5. 1 pH value
The pH value depends on the concentration of H+ in water. The higher the concentration of H+, the lower the pH value. PH value is a comprehensive physical and chemical index to measure the acid-base properties of aqueous solution, which has an important influence on the existing forms of chemical elements in aqueous solution and the interaction between groundwater and surrounding rocks. The pH value of the aqueous solution is restricted by many factors, mainly including the chemical composition of the solution, temperature and pressure (especially the partial pressure of gases such as CO2 and H2S).
Natural groundwater can be changed from strongly acidic water with a pH of 0.45 ~ 1 to strongly alkaline water with a pH of10 ~1.5, but most groundwater has a pH of 6 ~ 8.5. The lowest pH value (0.45 ~ 3) is generally related to the presence of free sulfuric acid or free hydrochloric acid in water. Water with pH value of 3 ~ 6.5 may be related to organic acids and carbonic acid gas (CO2) in addition to free sulfuric acid. Neutral and weakly alkaline water (pH = 6.5 ~ 8.5) is characterized by containing Ca(HCO3)2 and Mg(HCO3)2, and the pH value increases.
China's "Hygienic Standard for Drinking Water" stipulates that the pH value of drinking water should be between 6.5 and 8.5, and the pH value within this range will not affect human health. If the pH value of water is too high, it will lead to the precipitation of dissolved salts in water, worsen the sensory morphology of water and reduce the disinfection effect of chlorination. If the pH value of water is too low, it will cause strong corrosion of water and enhance the dissolution of metals (iron, lead, aluminum, etc.). ) by water.
4. 1.5.2 redox potential
The redox potential (Eh) is a comprehensive physical and chemical index representing the redox state of water, and the unit is V or mV. The gas, inorganic matter, organic matter and microorganism in natural water constitute a complex redox equilibrium system, and redox potential is the performance and result of this effect. The redox conditions of water body have an important influence on the existing forms of elements and their migration, enrichment and dispersion. Some elements have strong migration ability in oxidizing environment, while others are easier to migrate in reduced water. The redox potential of water is very sensitive to the change of environmental factors, and the changes of temperature, pH value and dissolved gas content will have great influence on it. Therefore, the Eh value is generally measured by special field instruments.
4. 1.5.3 Total dissolved solids
Total dissolved solids (TDS) refers to the total amount of dissolved components in water, including ions, molecules and complexes in water, but excluding suspended solids and gases. The total dissolved solids can be obtained by evaporating water at 105 ~ 1 10℃ and weighing the total amount of dry residue in mg/L or g/L. Besides direct measurement, the contents of all dissolved components (except dissolved gas) can be added up according to the results of water quality analysis, and half of HCO-3 content can be subtracted. The reason for subtracting half of HCO-3 content here is that about half of HCO-3 is converted into CO2 gas and H2O, and lost in the process of water sample evaporation. The reaction is as follows:
Introduction to groundwater science
According to the chemical dose relationship of the reaction equation, when 2mol HCO-3 is decomposed into CO2-3, 1molCO2 gas and 1molH2O will be produced, which will be lost in the evaporation process, and the lost molar mass is basically half that of HCO-3.
In addition to HCO-3, nitric acid, boric acid and organic matter may also be lost. On the contrary, some crystallization water (such as gypsum CaSO4 2H2O) and adsorbed water may remain in the dried residue. Therefore, there are often some slight differences between the measured and calculated values of TDS.
Mineralization is a term commonly used by Chinese scholars in the past, and its meaning is the same as that of total dissolved solids. The concept of salinity originated from the former Soviet Union and hardly appeared in the literature of other countries. In recent years, total dissolved solids have also been adopted by water supply, environment and other related departments in China.
4. 1.5.4 Salt content
Salinity refers to the total amount of each component in water, usually in mg/L or g/L. This index is a calculated value, which is different from the total dissolved solids because it does not need to subtract half of HCO-3 content. Salt content is often used to evaluate irrigation water quality and calculate the transportation of weathered products from rivers to the sea. In the study of marine hydrochemistry, salinity is often used instead of salinity. Salinity means that the content of all components in seawater accounts for one thousandth of the weight of water, expressed in ‰.
4. 1.5.5 hardness
The hardness of water reflects the total content of multivalent metal ions in water, including Ca2+, Mg2+, Sr2+, Fe2+, Fe3+, Al3+, Mn2+, Ba2+ and so on. Compared with Ca2+ and Mg2+, the content of other polyvalent metal ions in natural water is generally less, so the hardness of natural water is often mainly caused by Ca2+ and Mg2+. Hardness is usually expressed as the mg/L number of CaCO3, which is equal to the sum of the mg equivalent concentrations of all multivalent metal ions in water multiplied by 50(CaCO3 equivalent). In the past, China always expressed the hardness of water in German degrees. Because German degree is an illegal unit of measurement, the mg/L number of CaCO3 (legal unit of measurement) is used to represent hardness.
According to its hardness, water can be divided into soft water, slightly hard water, hard water and extremely hard water, as shown in Table 4. 1.
Table 4. 1 hardness classification of water
Hardness can be divided into total hardness, carbonate hardness and noncarbonate hardness. The total hardness is the sum of multivalent metal ions in water, which is expressed by the mg/L number of CaCO3. Carbonate hardness refers to the hardness that can be combined with CO2-3 and HCO-3 in water. When enough CO2-3 and HCO-3 are combined in water, the hardness of carbonate is equal to the total hardness. When CO2-3 and HCO-3 in water are insufficient, the hardness of carbonate is equal to the sum of mg equivalents of CO2-3 and HCO-3 multiplied by 50, that is, the total amount of CO2-3 and HCO-3 in water expressed in mg/liter of CaCO3. Carbonate hardness is usually called temporary hardness, because this part of hardness can be combined with CO2-3 and HCO-3 in water. When water boils, CaCO3 and other precipitates are formed and removed. The difference between total hardness and carbonate hardness is called noncarbonate hardness or permanent hardness, which refers to the total amount of multivalent metal cations combined with Cl-, SO2-4 and NO-3 in water, which cannot be removed after water is boiled.
The hardness of water usually varies greatly in different areas. Generally speaking, the hardness of surface water is less than that of groundwater. The hardness of groundwater often reflects the lithology of the stratum it contacts. When the topsoil is thick and limestone exists, the hardness of water is generally high, while soft water generally appears where the topsoil is thin and limestone is scarce or nonexistent.
The hardness of water has a certain influence on daily life and industrial water use. For example, hard water can react with soap to reduce the formation of foam and reduce the washing effect. High hardness water is easy to form scale in boilers and hot water pipes, which increases oil consumption, reduces thermal efficiency and blocks pipes. In recent years, it has also been found that the incidence of cardiovascular diseases is negatively correlated with the hardness of water, that is, the lower the hardness of drinking water, the higher the incidence of cardiovascular diseases.
4. 1.5.6 BOD
Biological oxygen demand (abbreviated as BOD) refers to the total amount of oxygen consumed by microorganisms in the process of degrading organic matter in water, expressed in mg/L, which is actually a substitute index reflecting the content of biodegradable organic matter in water. The higher its value, the more organic pollutants in the water. This index is often used to determine the pollution degree of domestic and industrial wastewater, as well as the pollution degree of surface water or groundwater, but it is rarely used for unpolluted natural groundwater.
BOD measurement is essentially a biodegradation process, and microorganisms degrade a certain amount of organic matter into carbon dioxide, water and so on. And measure the total amount of oxygen consumed in this process. The degree of completion of this process is usually determined by temperature and time. Theoretically, it takes a long time for organic matter to be completely oxidized. In order to shorten the detection time and ensure the comparability of BOD values, BOD is usually calibrated by the result of 5 days' culture at 20℃, which is called 5-day biochemical oxygen demand and is recorded as BOD5. Generally speaking, BOD5 has accounted for a considerable proportion in the total BOD, and it can account for 70% ~ 80% of the total BOD for domestic and industrial wastewater, which basically meets the needs of reflecting the organic matter content in water.
4. 1.5.7 chemical oxygen demand
Chemical oxygen demand (COD for short) refers to the amount of oxygen consumed by using chemical oxidants to oxidize organic substances in water and reduce inorganic substances in water, and the unit is mg/L. In the process of determining COD, no matter whether organic substances can be biodegraded or not, they are oxidized into carbon dioxide and water by oxidants. So COD is generally greater than BOD. The biggest disadvantage of COD determination is that it can't distinguish biodegradable organic matter from non-biodegradable organic matter, and it can't provide any information about the speed at which degradable organic matter reaches a steady state under natural conditions. Its advantage is that the determination time is short, only about 3 hours, so in many cases COD is used instead of BOD. When a large number of COD and BOD data are accumulated at the same time and the correlation between them is established, the COD data can be explained by BOD value.
Potassium permanganate and potassium dichromate are usually used as oxidants in the determination of chemical oxygen demand. The determination results of potassium permanganate method and potassium dichromate method are expressed by CODMn and CODCr respectively.
4. 1.5.8 total organic carbon
Total organic carbon (TOC) is the total amount of various forms of organic carbon in water, which is expressed in mg/L. Due to the variety of organic matter in water, it is not yet possible to separate and identify all of them. TOC is a comprehensive index for rapid detection. It represents the total amount of organic matter in water by the amount of carbon, which can be determined by measuring CO2 produced by high-temperature combustion, or by using a special TOC instrument. Compared with COD and BOD, TOC is a direct measurement index, not a substitute index, with high test accuracy, which can better reflect the total amount of organic matter in water. Because of the complexity of the traditional combustion method, it is difficult to eliminate the interference of inorganic carbon. When the content of organic carbon in water is low, the accuracy of the test results is poor, and there are few TOC data in the previous water quality analysis results. With the gradual popularization of TOC instruments and the reduction of testing costs, TOC testing will be more and more used in water quality analysis.
4. 1.5.9 alkalinity
Alkalinity is a comprehensive index to characterize the ability to neutralize acid in water. The alkalinity of natural water is mainly caused by weak acid salts in water. Of course, weak alkali and strong alkali also contribute to some extent. Generally speaking, carbonate and bicarbonate are the main components of alkalinity. Other weak acid salts, such as borates, silicates and phosphates, are usually contained in small amounts. A few salts formed by organic acids (such as humic acid) will also affect the alkalinity of natural water. Although there are many substances that affect the alkalinity of natural water, the alkalinity of water is mainly caused by three substances: hydroxide, carbonate and bicarbonate.
The alkalinity is generally determined by titration with strong acid standard solution such as sulfuric acid, and expressed in mg/L of CaCO3.
The alkalinity caused by carbonate and bicarbonate is usually called carbonate alkalinity. Carbonate alkalinity can be calculated by multiplying 50(CaCO3 equivalent) by the sum of mg equivalent concentrations of CO2-3 and HCO-3 according to the water quality analysis results.
4. 1.5. 10 acidity
Acidity is a comprehensive index of water and alkali capacity. The acidic substances in water can be divided into three categories: ① strong acids, such as HCl, HNO3, H2SO4, etc. ② Weak acids, such as CO2, H2CO3, HCO-3 and various organic acids; ③ Strong acid and weak base salts, such as FeCl3 _ 3 and Al _ 2 (SO _ 4) _ 3. The total neutralization ability of these substances to strong alkali in water is called total acidity. The total acidity is different from the concentration of H+ in water. The concentration of H+ refers to the amount of H+ in water in the state of free ions, while the total acidity indicates the total amount of H+ that can react with strong alkali during neutralization, including ionized and about to ionize. The amount of ionized H+ is called ionic acidity, and its negative logarithm is the pH value of aqueous solution. Like alkalinity, acidity is usually expressed in milligrams per liter of CaCO3 (Shen et al., 1993).