Spatial and temporal variability analysis of NO3-N concentration in shallow groundwater in Bay and Los Banos, Laguna [Philippines]

The spatial and temporal variabilities of NO3-N concentration in shallow groundwater in selected agricultural and residential areas in Bay and Los Banos, Laguna were analyzed in this study. The NO3-N concentration in selected shallow well was monitored for the whole wet cropping season. The spatial variation of NO3-N concentration was evaluated using three methods of interpolation, namely: inverse distance weighted, spline and kriging interpolation technique. The influence of rainfall prior to sampling on the amount of NO3-N concentration detected was also evaluated. The spatial variability of NO3-N over the sampling area using the inverse distance method generated maps that showed local peak caused by a single extreme value of NO3-N observed in a well located in a residential area in Bay. This may also be an indication of possible local contamination in the area. Spatial variability maps using inverse distance method showed that the wells with high NO3-N concentration have less influence on the surrounding wells. On the other hand, the spline method and kriging technique using unviersal linear variogram, generated maps that contained negative values. This may be due to the uneven distribution of the sampling points throughout the area. The generation of negative values indicate the limitations of these interpolation techniques. Although the inverse distance method generated maps with no negative values it did not show significant difference of the spatial variability of NO3-N concentration between agricultural and residential areas. Overall, no well-defined pattern of spatial variability of NO3-N was observed in both agricultural and residential areas. Temporal analysis of NO3-N concetration showed no well-defined pattern and fluctuated throughout the wet cropping season. These fluctuations mau have been influenced by such factors as the crop growth stage and the amount and timing of rainfall occurrences. Efficient consumption of the nitrogen fertilizer that was applied during the crop growing season may have lessened the amount of nitrate leached. Temporal variations in nitrate concentrations detected may have also been due to the agricultural activities done in the area like land preparation which increased infiltration occurring over the area causing groundwater recharge and by the amount of rainfall. However, the fluctuations of NO3-N concentrations with time may have been governed by the variations of the depth and timing of rainfall occurrences, the soil moisture conditions and aquifer hydraulic characters. The average NO3-N concentration within the agricultural areas exhibited negative correlation values with the 3-day, 7-day and 14-day cumulative rainfall prior to sampling whereas average NO3-N within residential areas showed positive correlatrion with the three sets of cumulative rainfall. The monthly average NO3-N concentration showed a weak but positive correlation with the 7-day cumulative rainfall with a correlation coefficient of 0.1833. A very weak lineat relationship was exhibited by the average monthly NO3-N concentration with the 3-day and 14-day cumulative rainfall with correlation coefficient values of 0.0173 and 0.0707, respectively. The negative correlation between NO3-N detected in the groundwater samples. In terms of nitrogen fertilizer loss, estimates based on observed NO3-N concentration and average fertilizer application rates showed that only 1.58 to 1.78 percent of the applied N-fertilizer was lost to leaching. The highest loss of nitrogen fertilizer, 1.78 percent occurred during the second application, which was done during the crop maturity stage. These relatively low values suggest high fertilizer application efficiency. The concentrations of NO3-N detected in shallow groundwater in Bay and Los Banos area in this study show that groundwater quality in the shallow equifer in these areas is still not an object of serious environmental concern and is still suitable for domestic consumption, as the NO3-N concentration detected are all below the drinking water standard of 10 mg/L set by th Department of Health. However, the well in barangay [village] Calo is an exemption due to consistently high observed NO3-N concentrations with an observed maximum of 9.40 mg/L.