Study site and sampling The study was conducted at the Banglades Institute of Nuclear Agriculture farm Mymensingh and at its sub-stations located Ishurdi, Rangpur, Magura, Comilla and Satkhin having different agro-ecological zones. At each location, water samples were collect, from the existing deep tubewells (DTWs) or shallow tubewells (STWs) or from the san sources of the nearby National Agricultural Research System (NARS) Institutes. Year round times were chosen to monitor the groundwater movement and its seasonal changes in chemical. composition as influenced by pumpage during the dry season and recharge by rainfall during the monsoon. Quality monitoring was stepwis done and the duration of monitoring for a' locations varied from two to four years for all the parameters except iron. Iron was monitor, once in May 2003. Thus, sampling period Wa! from July 1999 to June 2003. Water samples were collected in dry and clean plastic containers. Prior to sampling, the containers were rinsed well with sampling water. The samples were collected only after the tubewells had pumped for at least 30 minuets to avoid stagnant or contaminated water sampling. After collection of water, the containers' mouths were sealed tightly. The containers were kept airtight and labeled properly to avoid complication in identification. The water samples were then carried to the Bangladesh Institute of Nuclear Agriculture laboratory and preserved in a cool room for analysis.
Chemical analysis: The groundwater samples were chemically analyzed in order to find their suitability for irrigation purposes. The samples were analyzed for pH, Electrical Conductivity (EC), soluble cations including Calcium (Ca ++), Magnesium (Mg++), Sodium (Na+), Potasium (K+), Iron (Fe++) and Arsenic (As+); the anions including Carbonate (CO3 -), Bicarbonate (RCO3') and Chloride (Cl), pH, EC and As+ were determined by digital pH meter, conductivity bridge and field kit method, respectively. While Ca ++, Mg++, Na+ and K+ were determined by atomic ab orption spectrophotometer. The rest of the samples were analyzed according to the standard method suggested by Richards (1954).
Water quality criteria: Generally the most accepted criteria for judging the quality of water are: i) the total salt concentration measured by EC, ii) relative proportion of cations expressed by SAR and iii) HCO3 - and boron (B) contents. The suitability for irrigation water (SIW) is expressed as: SIW =f (QSPCD) Where, Q is quality of irrigation water, S is soil type, P is salt tolerance characteristics of the plant, C is climate and D is drainage characteristics of the soil. Beside these, other factor like water table depth, presence of soil hard pan, calcium carbonate content in the soil and potassium and nitrate ions also indirectly influence the suitability of irrigation water.
Toxicity problem: It occurs when certain ions mainly chloride, sodium or boron are taken up by plants from soil and water and accumulate to concentrations high enough to cause crop damage or reduced yields. Crop damage depends on the uptake and crop sensitivity, which often occurs at relatively low ion concentration for highly sensitive crops. The permanent, perennial-type crops are more sensitive. Citrus crops are sensitive to sodium and chloride. Toxicity also accompanies salinity and reduces water infiltration rates.
Miscellaneous problems: Several other problems are also associated with low quality of irrigation water. Some of these are: i) Excessive vegetative growth, lodging and delayed crop maturity ii) Deposition of bicarbonate, gypsum and iron as stains on fruits and leaves iii) Irrigation water with a pH outside the normal range may cause a nutritional imbalance or may contain a toxic ion iv) Corrosion and encrustation in pumps, tubewells and other problems in irrigation equipment v) Reduced infiltration rate in soils leading to water stagnation in crop field and infestation of various crop diseases.