Remediation effects of compound well on nitrate pollution in groundwater

Abstract: Nitrate pollution has posed a great threat to groundwater near farmlands, due mainly to the long-term agricultural fertilization and soil microorganisms. Nitrates with strong migration ability have entered the zone of saturation along with water movement. High concentrations of nitrates are directly detrimental to the safety of groundwater source areas. In this study, three systems A, B, and C were constructed to explore an in-situ bioremediation technology for the detection of nitrate-contaminated groundwater around farmland. Every system consisted of wells, storage tanks, and peristaltic pumps. System A was used to simulate a tube well with a diameter of 7 cm. System B and C were used to simulate compound wells, where there were a large well with a diameter of 14 and a tube well with a diameter of 7 cm. Every tube well was filled with fine sand in the same volume and height, where the height of fine sand was 12cm. The large diameter wells in system B and C were filled with fine sand with the heights of 4 cm and 8 cm, respectively. Both tube wells and large wells were used to simulate the complete penetration wells. The bottom of the wells was sealed, where water flowed in from the side walls. The peristaltic pumps were installed on the top of wells to simulate water pumping. The volume ratio of the reaction medium was 1:2:3 in three systems. The ratio of hydraulic retention time was also 1:2:4 under the same flow rate. The biofilm was naturally domesticated. Ethanol was used as the carbon source. A total of 6 groups were set in the reaction stage, including 25, 50, 75, 100, 125 and 150 mg/L, according to the concentration gradients of nitrate nitrogen. The test results showed that the microorganisms in the reaction medium could basically select the dominant strains within one day when the nitrate concentration increased instantaneously, leading to match the changes in nitrate loading conditions for the better denitrification capacity. In-situ bioremediation systems were also constructed with compound wells including tube wells and large diameter wells, in order to repair nitrate-contaminated groundwater and the nitrate loading of every system. The nitrate loadings of three remediation systems A-C were 75-100 mg/L, 100-125 mg/L, and 125-150 mg/L at the flow rate of 0.26 m/d. The removal rate of remediation systems reached more than 95% within nitrate loading. There was no accumulation of nitrite and excessive ammonia nitrogen, indicating the feasibility of repair systems with compound wells. Groundwater mining and remediation were carried out simultaneously without the need to cut off agricultural non-point source pollution, indicating high security for groundwater source area. In addition, a combination of tube wells and large diameter wells can be installed to compound wells with a relatively simple well drilling (including phreatic water and confined water). The diameter of wells and height of the reaction medium can be adjusted for better removal of nitrates, according to the thickness of the aquifer, the amount of water demand, and the level of nitrate-contaminated groundwater. Physical test models were used to determine the nitrate loading intervals of three remediation systems. In the future work, the hydrogeochemical model will be established to accurately determine the maximum nitrate loadings of repair systems with compound wells, together with the influence of medium heterogeneity on the remediation performance.