Water Quality Issues in Dry Regions (With Solutions) | Water Management

The scarce rainfall, high temperature and evaporation lead to limited water resource in the dry arid and semiarid regions. Dry regions suffer from both the problems of quantity and quality of water. In most part of the dry region, groundwater is either saline or having high nitrates and fluoride content especially in States of Rajasthan, Gujarat, Haryana, Punjab, Andhra Pradesh and Karnataka.

The other water related problems in these regions include organic and metallic pollution of rivers, ponds and groundwater due to disposal of untreated effluents coming from industries like textile. In this article, main focus will be on groundwater quality especially salt, fluoride and nitrate contamination. Obviously, groundwater is the major source of drinking water and most of the drinking and irrigation water demands are met by groundwater, especially in these regions.

The quality aspects of groundwater in the country are being monitored by the Central Ground Water Board (CGWB) through a network of about 15500 groundwater observation wells all over India and about 1375 in Rajasthan.

The results of this monitoring and affected area are discussed below:

Issue # 1. Groundwater Salinity:

The term “salinity” refers to the total dissolved concentration of inorganic ions in water. Total salinity may include hundreds of different ions; however, relatively few are most important: chloride (CI^–), sodium (Na), nitrate (NO₃^–), calcium (Ca), magnesium (Mg⁺²), bicarbonate (HCO₃ ^–) and sulphate (SO₄^-2).

The concentrations of boron (B), bromide (Br), iron (Fe), and other trace ions can be locally important. Total salinity is generally measured as total dissolved solids (TDS) in mg/1 or electrical conductivity (EC) expressed in units of deci Siemen per metre (dSm^-1).

The classification of water based on salinity is given in Table 10.3.

a. Status of Groundwater Salinity in Dry Regions of India:

The high salt content in the groundwater of arid and semi-arid regions of Rajasthan, Haryana, Punjab and Gujarat are major water quality issue in these regions. According to Central Ground Water Board (CGWB), about 2 lakh sq. km area in India has been estimated to be affected by saline water of EC in excess of 4000 dS m^-1 and there are several places in Rajasthan and southern Haryana where values of electrical conductivity of groundwater is greater than 10000 dS m^-1 at 25°C making water non-potable and unsuitable for irrigation.

BIS has recommended a drinking water standard for total dissolved solids a limit of 500 mg l^{– 1} (corresponding to about EC of 750 µS cm^-1 at 25°C) that can be extended to a TDS of 2000 mg/l (corresponding to about 3000 µS cm^-1 at 25°C) in case of no alternate source of water. Water having TDS more than 2000 mg l^-1 is not suitable for drinking uses.

However, in some cases, relatively high values of EC in excess of 3000 µS cm ^-1 are observed in many parts of the country especially in dry regions of Rajasthan, Punjab, Haryana, Gujarat etc. About 30 districts of Rajasthan, 21 districts of Gujarat, 16 districts of Andhra Pradesh and 15 districts of Haryana are severely affected by groundwater salinity with EC more than 3000 µS cm^-1 (Table 10.4).

b. Causes of Groundwater Salinity:

The occurrence of saline groundwater in coastal area is due to seawater intrusion while in terrestrial region especially dry region is geogenic as well as anthropogenic. There are several mechanisms by which water trapped in sedimentary rocks can be altered into saline water. One of these is the solution of sediments and rocks.

The natural chemical composition of groundwater is influenced predominantly by type and depth of soils and subsurface geological formations through which groundwater passes. Some rocks dissolve very easily; groundwater in these areas can naturally be very high in salinity. Numerous investigators have noted that water within sedimentary strata becomes increasingly saline with an increase in depth.

In general, the sequence noted is sulphate-rich water near the surface, saline bicarbonate water at an intermediary level and more concentrated chloride water at greater depths. Human activities also affect salinity levels in ground and surface water. Application of synthetic fertilizers, manures, and wastewater treatment facilities can all contribute salt to surface and groundwater through runoff and leaching. Inland salinity is also caused due to practice of surface water irrigation without consideration of groundwater status.

The gradual rise of groundwater levels with time has resulted in water logging and heavy evaporation in semi-arid regions leading to salinity problem in command areas. As per recent assessment, about 2.46 m ha of the area, under surface water irrigation projects, is water logged or threatened by water logging in India.

Issue # 2. Impact of Saline Water on Crops and Soil:

The application of saline irrigation water leads to reduction in crop yield through direct impact on crop growth and deterioration of soil quality which, in turn, may affect the suitability of the soil as a medium for plant growth. The primary effect of high EC water on crop productivity is the inability of the plant to compete with ions in the soil solution for water (physiological drought).

The higher the EC, the less water is available to plants, even though the soil may appear wet. Excessive salinity in root zone reduces plant growth primarily because it requires extra energy to acquire water from the saline soil of the root zone and to make the biochemical adjustments necessary to survive under stress.

This energy is diverted from the processes, which lead to growth and yield. Yield reductions occur when the salts accumulate in the root zone to such an extent that the crop is no longer able to extract sufficient water from the salty soil solution, resulting in a water stress for a significant period of time. If water uptake is appreciably reduced, the plant slows its rate of growth.

This effect is manifested as nearly equivalent reductions in the transpiration and growth rates (including cell enlargement and the synthesis of metabolites and structural compounds). The symptoms showed by plants grown in saline condition are similar in appearance to those of drought, such as wilting, or a darker, bluish-green colour and sometimes thicker, waxier leaves. Symptoms vary with the growth stage, being more noticeable if the salts affect the plant during the early stages of growth.

In some cases, mild salt effects may go entirely unnoticed because of a uniform reduction in growth across an entire field. Certain ions (sodium, chloride, or boron) from soil or water accumulate in a sensitive crop to concentrations high enough to cause crop damage and reduce the yields.

Salinity of water is due to presence of ions and concentration and balance of different ions determine the adverse impact of saline water on the soil. The adverse impact on soil due to irrigation with high salt content includes reduction in infiltration rate, crusting and ion toxicity. An infiltration problem related to water quality in most cases occurs in the surface few centimetres of soil and is linked to the structural stability of this surface soil and its low calcium content relative to that of sodium. High salinity water leads to increase infiltration while low salinity water or water with high sodium to calcium ratio decreases infiltration. Both factors may operate at the same time. When a soil is irrigated with high sodium water, a high sodium surface soil develops which weakens soil structure.

The surface soil aggregates get dispersed to much smaller particles which clog soil pores. The problem may also be caused by an extremely low calcium content of the surface soil. In some cases, water low in salt can cause a similar problem but this is related to the corrosive nature of the low salt water and not to the sodium content of the water or soil. In the case of low salt water, the water dissolves and leaches most of the soluble minerals, including calcium from the surface soil.

Issue # 3. Fluoride Contamination:

Human consumption of fluoride has both beneficial and detrimental effects on health. The intake of fluoride within permissible limits is known to be beneficial for human health in the production and maintenance of healthy teeth and bones, while excessive intake of fluoride causes dental and skeletal fluorosis, which is a chronic disease manifested by mottling teeth in mild cases and softening of bones and neurological damage in severe cases.

BIS has recommended an upper desirable limit of 1.0 mg l^-1 of F as desirable concentration of fluoride in drinking water, which can be extended to 1.5 mg l^-1 of F in case no alternative source of water is available. Water having fluoride concentration of more than 1.5 mg l^-1 is not suitable for drinking purposes. Fluoride in drinking water has appeared as a serious problem, and around 200 million people, from 25 nations the world over, are affected by fluorosis. In India, about 66.62 million people are at risk of consuming fluoride-contaminated water in 19 States of India including 6 million children below the age of 14 years.

i. Status of Groundwater Fluoride Contamination in Dry Regions of India:

Fluoride contamination in groundwater is endemic in India. High concentration of fluoride in groundwater beyond the permissible limit of 1.5 mg/L is a major health problem in India. Nearly 90% of rural population of the country uses groundwater for drinking and domestic purposes and due to excess fluoride in groundwater a huge rural population is threatened with health hazards of Fluorosis.

According to CGWB, the fluoride content in major part of the country is found to be less than 1.0 mg l^-1, however, there are several locations mainly in the States of Andhra Pradesh, Gujarat, Karnataka, Madhya Pradesh, Rajasthan, Chhattisgarh, Haryana, Orissa, Punjab, Uttar Pradesh, West Bengal, Bihar, Delhi, Jharkhand, Maharashtra, and Assam where the fluoride in groundwater exceeds 1.5 mg l^-1. Most of the contaminated sites fall under arid and semi-arid regions of the country. Among the different States, Rajasthan is one of the States in the country where a higher level of fluoride is reported and almost all the districts (34) are affected by fluoride contamination in groundwater (Table 10.5).

Recent case studies indicate that about 95% of sites of this region contain a higher fluoride level in groundwater than the maximum permissible limit as decided by the Bureau of Indian Standards. The contamination is mainly due to presence of accessory minerals, fluorite and apatite in the rock mineral assemblage wherein the groundwater is stored, as well as the environmental factors such as precipitation and evaporation.

The cereals, pulses and vegetables grown in fluoride-endemic areas have also shown higher contents of fluoride when compared with those grown in the areas where fluoride level is 0.1 – 0.5 mg l^-1. Amongst other vegetables, spinach has the highest fluoride content (29.15 mg kg^-1). The milk of cow and goat also have fluoride content ranging from 0.41 to 6.87 mgl^-1, whereas, in non-fluoride affected areas, the cow milk has 0.1 mg l^-1of fluoride in it.

ii. Causes of Fluoride Contamination:

The contamination of groundwater with fluoride is geogenic in nature while some human activities also enhanced the chances of contamination. Naturally occurring fluorides in groundwater are result of the breakdown and dissolution of fluoride-containing rock minerals by water while artificially high soil fluoride levels can occur through contamination by application of phosphate fertilizers or sewage sludge, or from pesticides. The fluoride compounds added to soils by pollution are usually readily soluble.

The occurrence of fluoride in natural water is affected by the type of rocks, amount of soluble and insoluble fluoride in source rocks, oxidation- reduction process, climatic conditions, nature of hydro geological strata and time of contact between rock and the circulating groundwater. Presence of other ions, particularly bicarbonate and calcium ions also affects the concentration of fluoride in groundwater.

The most common fluorine-bearing minerals, which constitute natural source for fluoride in drinking water, are fluorite, apatite, rock phosphate and topaz. Fluorite (CaF₂) is a common fluoride mineral. This mineral has a rather low solubility and occurs in both igneous and sedimentary rocks. Most fluorides are sparingly soluble and are present in natural water in small amounts.

Since the presence of fluoride-bearing minerals in host rocks and their interaction with water is considered to be the main cause for fluoride contamination in groundwater. High fluoride concentrations in groundwater associated with igneous and metamorphic rocks such as granites and gneisses have been reported from India, Pakistan, West Africa, Thailand, China, Sri Lanka, and South Africa. In tropical countries, fluoride in drinking water is a major chemical threat because in hot tropical part of the world, people consume more water, and consequently, the risk of fluoride accumulation increases.

Major and widespread cause of groundwater quality deterioration is heavy pumping, which may cause the migration of more highly mineralized water from surrounding strata to the well. Intensive and long-term irrigation causes weathering and leaching of F from the soils/weathered rocks. Easy accessibility of circulating water to the weathered products during irrigation dissolves and leaches the minerals, including fluorine, contributing fluoride to the surface water and groundwater. Weathering of alkali, silicate, igneous and sedimentary rocks contribute a major portion of fluorides to groundwater. Occurrence of alkaline soil around the fluoride contaminated water sources is the outstanding feature of the fluoride-affected areas.

Issue # 4. Nitrate Contamination:

Nitrate is essential for plants as well as animals. Nitrate form is most useful form of nitrogen as is biologically available to plants, and is, therefore, valuable fertilizer. Though nitrate is considered as relatively non-toxic to human, a high nitrate concentration in drinking water is an environmental health concern. The specified limits are not to be exceeded in public water supply.

As per the BIS Standard for drinking water, the maximum desirable limit of nitrate concentration in groundwater is 45 mg l^-1 with no relaxation. If the limit is exceeded, water is considered to be unfit for human consumption. Nitrogen poisoning affects infants by reducing oxygen carrying capacity of blood by binding to hemoglobin (Hb; methemoglobin [Met Hb]), causing a condition referred to as methemoglobinemia. Methemoglobinemia is also commonly referred as “blue baby” syndrome because lack of oxygen lead to bluish colouration in skin of the infant especially nose tip.

Recent studies have revealed that nitrate can be endogenously reduced to nitrate, which can then undergo nitrosation reactions in the stomach with a mine and a mines to form a variety of N-nitroso compounds, which are mainly carcinogens. But due to intensive fertilizer application in the agricultural land and seepage from waste dumping site, increasing concentration of nitrate in groundwater is becoming global concern.

In India also, groundwater of many States are severely contaminated with nitrate, which has been discussed below:

a. Status of Nitrate Contamination in Dry Regions of India:

According to CGWB, in India, high concentration of nitrate (more than 45 mg l^-1) has been found in many districts of Andhra Pradesh, Bihar, Delhi, Haryana, Himachal Pradesh, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Orissa, Punjab, Tamil Nadu, Rajasthan, West Bengal and Uttar Pradesh. However, largely affected areas are arid and semiarid region mainly concentrated in Rajasthan, Gujarat and Karnataka (Table 10.6).

b. Causes of Nitrate Contamination:

Being negative charge, nitrate (NO₃^–) does not get adsorbed on negative soil colloids, and as a result, is highly mobile, and liable to leached down into the groundwater. For nitrate leaching to occur: nitrate must be present in the soil, the soil must be permeable for water movement, and water must be moving through the soil. Nitrate in a field may originate from many sources, including manures, composts, decaying plants, septic thinks, or from fertilizer. Geologic sources of fossil N can add significant amounts of nitrate to water in some regions.

However, the excessive use of nitrogenous fertilizer like urea which gets readily transformed into nitrate is major cause of non-point source of groundwater contamination. Water added in excess of the soil’s water-holding capacity carry nitrate and other salts downward. Any factor influencing soil moisture (such as rainfall, irrigation, evaporation and transpiration, depth to groundwater, soil texture, and rooting depths of plants) will impact nitrate movement. In general, more water infiltration results in nitrate moving deeper into the profile. Soil properties also have a major impact on the extent of nitrate movement.

However, the extent of nitrate movement to groundwater also depends on the underlying soil and bedrock conditions, as well as depth to groundwater. Fine-textured soils (high clay) are generally less susceptible to nitrate movement than sandy-textured soils because water permeability is much lower.

Due to coarse texture soil in arid and semiarid region especially Rajasthan, nitrate get easily leached down to groundwater making this zone highly susceptible for nitrate contamination. Water movement through soil cracks and macro pores (preferential flow) can be as much as twenty times higher than in the same soil without cracks.

This movement can allow nitrate to flush through soil more rapidly than might be expected. However, downward moving water may likely encounter many features such as perched water tables, geologic discontinuities, restrictive layers, and other barriers to nitrate movement.

In dryland conditions, nitrate leaching is likely insignificant during the summer season because plant uptake of water usually exceeds precipitation, preventing downward movement of water. However, in rainy season, the opposite is true, resulting in increased potential for nitrate leaching.

Solution for Use of Poor Quality Water in Dry Regions:

1. Managing Saline Groundwater:

The increasing scarcity of freshwater worldwide is forcing the farmers to use marginal quality water. However, the use of saline water for crop irrigation may lead to secondary salinization in soil especially in arid and semiarid regions where evapotranspiration exceeds rainfall. For the safe use of saline water, proper, planning and management is necessary.

The objectives of management practices to be followed for optimal crop production with saline water include the prevention of salt build-up to levels which limit the productivity of soils and the control of salt balances in the soil-water system, as well as minimising the damaging effects of salinity on crop growth.

The sustainable use of saline water in irrigated agriculture requires the control of soil salinity at the field level, a decrease in the amount of drainage water, and the disposal of the irrigation return flows in such a way that minimizes the side effects on the quality of downstream water resources.

The most important points to be considered during using saline water irrigation are:

(a) Suitability for Irrigation:

The first step in managing saline water is to evaluate its suitability for irrigation depending on soil type, crop and climatic condition. The FAO guideline can be widely applied in the irrigated lands of the arid and semi-arid regions and cover the range from sandy loam to permeable clay loam soils.

(b) Leaching of Added Salt:

To avoid secondary salinization of the soil due to irrigation with high salinity water, the added salts have to be washed out at least from the upper root zone. If the electrical conductivity of irrigation water is lower than 0.7 dS m^-1, losses of water by deep percolation that are generally higher than 15 percent of the amount of water applied, are sufficient to achieve an acceptable salt content in the soil solution. Whereas, when waters of higher salinity with values of EC between 0.7 and 3.0 dS m^-1and higher are applied, more leaching is required to maintain a long-term adequate salinity content.

In this case, percolation water can be insufficient in the whole irrigated field or in that part that receives less water and then a process of secondary salinization can emerge. For this, extra water (or rainfall) must, over the long term, be applied in excess of that needed for evapotranspiration and must pass through the root zone in a minimum net amount.

Therefore, to check if the average amount of percolation water satisfies the minimum leaching requirement to avoid soil salinization, a calculation of the leaching requirements is needed. This amount, in fractional terms, is referred to as the “leaching requirement”. Leaching requirement is defined as the minimal fraction of the total water applied that must pass through the root zone to prevent reductions in crop yield below the acceptable level. The leaching requirements for any acceptable yield (Y,) can be calculated using the equations.

(c) Subsurface Drainage:

To avoid accumulation of leached out salt into subsurface soil layer, subsurface drainage must be provided as outlet for the removal of salts that accumulate in the root zone in order to avoid excessive soil salinization. It must keep the water table sufficiently deep to permit adequate root development, to prevent the net flow of salt-laden groundwater up into the root zone by capillary forces and to permit the movement and operation of farm implements in the fields, which also provide good aeration and adequate water content in the root zone.

(d) Irrigation Methods:

Flood irrigation should be avoided and forms of irrigation that minimize matric stress, such as drip irrigation, can be used to minimize the harmful effects of irrigation with saline water.

(e) Salt Tolerant Crop:

Depending on the physiological and metabolic activity, some crops may be susceptible and some may be tolerant to soil salinity. So, choice of crop and their cultivar is necessary for getting better crop yield under saline water irrigation condition (Table 10.7).

2. Managing Fluoride Contaminated Water:

The negative health effect of consuming fluoride contaminated water has led to development of several technologies for removal of fluoride from drinking water and many are evolving. The popular technologies for the removal of fluoride from water include: coagulation followed by precipitation, membrane processes, ion exchange and adsorption.

All techniques have their own advantages and disadvantages. Adsorption techniques have been quite popular in recent years due to their simplicity, as well as the availability of wide range of adsorbents. Research has focused on various types of inexpensive and effective adsorption media, such as different clays, solid industrial wastes like red mud, spent bleaching earths, spent catalysts and fly ash, activated alumina, carbonaceous materials, bone charcoal natural and synthetic zeolites and other low-cost adsorbents, with various degrees of success.

In India, Nalgonda process developed by the National Environment Engineering Research Institute (NEERI), Nagpur and Activated Alumina process are recommended for removal of fluoride from drinking water. These technological options use aluminum salts as coagulant for removal of fluoride and a small amount of residual aluminum may remain in the treated water which may be toxic at higher concentration.

Fluoride selective ion-exchange resin is also offered by many manufacturers to remove fluoride in the raw water. However, this process is only effective if regeneration is closely monitored and executed regularly. In the absence of regeneration or timely change of resin, the treatment system with the presence of exhausted resin gives false sense of security to the users. Recently use of Reverse Osmosis (RO) technology has been reported highly efficient in removing fluoride from drinking water. Safe Water Network India installed many RO plants in India in the States of Telangana and Uttar Pradesh. RO plants of 1,000 liters capacity per hour are installed in selected 32 fluoride-affected villages of Warangal district of Telangana, where fluoride concentration in groundwater varied from 0.9-1.8 mg 1^-1. The treated water has 0.1-0.4 mg 1 ^-1 fluoride showing 90% removal efficiency.

3. Reducing Nitrate Contamination:

The excess and improper use of nitrogenous fertilizer in agricultural land is one of the factors that cause groundwater contamination with nitrate. However, with proper planning and management, the leaching of nitrate to groundwater or runoff to surface water can be easily reduced. One key practice for reducing leaching losses is to minimize the amount of nitrate present in the soil at any given time and to manage the irrigation water for avoiding leaching. To achieve the greater efficiency and minimum loss, the concept of ‘right source, right amount, right time, right place and right method’ should be followed in nitrogen and irrigation management.

The following points need to be taken into account for reducing nitrogen loss and nitrate leaching:

(a) Soil Test Based Fertilizer Application:

When calculating fertilizer rates, all sources of N available to the crop, including manures, legume input, organic matter and soil nitrate-N should be considered.

(b) Matching Time of Demand by Crop with Time of Fertilizer Application or Applying in Split:

Nitrate concentrations in soil should ideally be not more than as required to meet plant nutritional needs. Soil nitrate should be depleted as much as possible by the time harvest occurs to minimize loss between crops. The use of non-legume winter cover crops to recover residual soil nitrate can be effective in some situations.

(c) Type of Fertilizer:

Using an ammonium source of N fertilizer can temporarily limit initial N movement. Nitrification inhibitors can temporarily delay the appearance of nitrate. Controlled release fertilizers are also effective at limiting nitrate loss e.g. use of neem coated urea, nitrification and urease inhibitors

(d) Careful Management of Irrigation:

Since nitrate is leached down with percolating water, the presence of excess water should be avoided in root zone. Irrigation should be managed to meet the crop need, but not exceed the ability of the soil to hold the water on site. It can be achieved by applying irrigation water uniformly, only up to the water holding capacity of soil, irrigation scheduling, water should not be added until the crop has used the available moisture previously added in the root zone, water should added where plant roots can access, i.e. use of efficient irrigation system like drip and sprinkler.