Characterization of groundwater quality in selected tobacco-growing areas in Batac, Ilocos Norte [Philippines].

A field survey was conducted in selected tobacco-growing areas in Batac, Ilocos Norte [Philippines] to evaluate the quality of groundwater used for irrigation. A total of 40 shallow wells from barangays [villages] Naguirangan (11), Tabug (10), Palongpong (7), San Pedro (5), and Baoa West (7) were selected and monitored from February 1988 to June 1988. Electrical conductivity of groundwater samples (ECgw) in all barangays was observed to be high during April and May with Naguirangan having the highest mean EC value of 1.0 dS/m. Based on the salinity classification system, groundwater in all barangays were moderately to highly saline. No groundwater sample was classified as C1 (low salinity water). The percentage of samples in Naguirangan and Tabug classified as C3 (high salinity water) significantly increased to 95.2 percent during the third sampling date. Groundwaters in all areas were dominated by calcium and bicarbonate ions. The sodium adsorption ratio (SARgw) of the water samples (0.93-2.10) was lower than the critical SAR values (9-15) because of their high calcium and magnesium contents. Relationship between ECgw and sodium content yielded fairly high coefficient of determination for samples in Naguirangan and Tabug (R2 = 0.65) and in Palongpong, San Pedro and Baoa West (R2 = 0.67). Electrical conductivity values of soil saturation extracts (ECe) from all barangays were low which suggest that the maximum salt concentration is below the sampling depth (0-30 cm). Significant correlation between sodium adsorption ratio of soil saturation extracts (SARe) and groundwater samples (SARgw) were noted only in Naguirangan and Tabug. Sodium content of groundwater in these areas can also be used to predict SARe with fairly good results (R2 = 0.60). The occurrence of low SARe values was consistent with the low SAR of groundwater used for irrigation.