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D. Dissolved Solids, All waters in nature contain dissolved solids .Water is the universal solvent dissolving more different materials than any other liquid. Natural waters come in contact with soluble substances in many ways such as mere contact with its own basin, erosion at shore line, windblown materials, inflow of surface waters, inflow of seepage and other forms of subterranean waters and decay of aquatic organisms. Rain water contains 30 to 40 ppm of dissolved solids., Salts are composed of ions which in the solid form are held together by ionic forces. The strong ionization of the salts leads to the formation of hydrates with water in which the water acts as a dipole to which the ions are attached. The solubility of solid substances is strongly dependent on the pH and the redox potential in the water. It usually increases with temperature and is largely independent of pressure. Most substances dissolve either in the molecular form or dissociated into ions. Some important constituents such as humic acids, salicilic acid and ferric oxyhydrate are dispersed in colloidal form., a. Carbonate and Bicarbonate, The major ion contents vary in different fresh waters due to five factors, which are climate, geography, topography, biotic activity and time. These are not completely independent and they interact., Carbonate is the principal anion in most fresh-waters. Generally carbonate occurs as bicarbonate ion with calcium in water. Bicarbonate ion is customarily expressed as CO3 because evaporation of a known amount of calcium bicarbonate solution leaves only the carbonate of calcium to be weighed. During evaporation, gaseous CO2 and water are lost, from bicarbonate ions, converting them to a lesser weight of carbonate., Alkalinity is usually a measure of carbonates. There are various compounds of carbonates with calcium, such as calcite or aragonite which have the same chemical formula (CaCO3), but are crystallized differently. Aragonite precipitates from thermal waters and is contained especially in the shells of freshwater mollusks. Magnetite, the carbonates of magnesium (MgCO3) and dolomite, a double carbonate of calcium and magnesium, Ca Mg (CO3)2 are also relatively common. Carbonates of barium (BaCO3) and strontium (SrCO3) also occur. CaCO3 is insoluble except in the presence of acid. With carbonic acid, it becomes Ca (HCO3)2. Because of this, it seems reasonable to express alkalinity titrations in terms of bicarbonate ions, but on the other hand, Ca(HCO3)2 is very unstable and when water is evaporated to determine its contained dissolved salts, the bicarbonate of calcium is destroyed and only carbonate remains., b. Dissolved inorganic solids, i) Nitrogen compounds, Nitrogen occurs in natural waters in the form of numerous compounds, in inorganic form as nitrate, nitrite and ammonium and in organic form as intermediate stages of microbial protein decomposition. The most important inorganic nitrogen compounds in water are nitrate and ammonia. Natural waters contain some ammonium salts. Ammonium carbonate is probably the common form., ii) Phosphorus compounds, Free phosphorus does not occur in nature, but in the form of phosphates it is abundant. Inorganic phosphorus compounds usually occur in dissolved form only in small amounts in natural waters, often only as traces. Total phosphorus in lake water includes two components. One is soluble phosphorus which is the phosphate form and another one is organic phosphorus which is contained in plankton organisms and other organic matter in the water. As an essential nutrient for primary producers, phosphorus thus acts more often than nitrogen as the growth limiting factor. The natural inorganic phosphate content originate from atmospheric precipitation as well as from various phosphate containing rocks especially apatite, which are flushed into the lake by tributary streams. In lakes and flowing waters three phosphate fractions occur concurrently : soluble inorganic phosphate as orthophosphate (PO4) and polyphosphate, soluble organic phosphate and particulate organic phosphate (organisms or detritus). These fractions make up to total phosphate content. The losses of phosphorus occur throughout flowing water which removes both soluble and organic form. It may also occur through removals of fish, mollusks, water plants and other organisms.