In situ denitrification of nitrate rich groundwater in Marydale, Northern Cape
South Africa is a water scarce country and in certain regions the quantity of surface water is insufficient to provide communities with their domestic water needs. In many arid areas groundwater is often the sole source of water. This total dependence means that groundwater quality is of paramount importance. A high nitrate concentration in groundwater is a common cause of water being declared unfit for use and denitrification has been proposed as a potential remedy. In groundwater of the Marydale district in the Northern Cape Province, nitrate levels are high enough to be of concern for domestic and livestock consumption. A review of the literature indicates that bacterial denitrification of groundwater can be achieved in situ by using a suitable energy substrate. The technology has been tested elsewhere in the world but more certainty is needed on whether it is a feasible option for local groundwater remediation using local, cost-effective energy substrates and exploiting bacterial populations present naturally in the regolith. The objective of this study was to perform denitrification experiments by laboratory incubation using soil and groundwater samples collected in Marydale in order to determine; 1) The effectiveness of different carbon sources; 2) The effect of using soil sampled at different depths; 3) The effect of C:N ratio of the carbon substrate; and 4) The quality of resultant water. Various experiments were set up using 10 g soil and 40 mL groundwater with different concentrations of carbon sources (sawdust, glucose, maize meal and methanol). All experiments were done under a nitrogen atmosphere to exclude oxygen and temperature was kept constant at 23 °C. Indicator parameters were selected based on literature review, and major cations and anions and some metals were analysed for initially and at selected times during each experiment to evaluate whether major ion chemistry was changing over time. Parameters analysed in supernatant solutions after varying periods of time to indicate progress of denitrification and reduction included nitrate, nitrite, sulfate, alkalinity, chloride, acetate, basic cations, ammonium, pH, electrical conductivity, dissolved organic carbon, heteThe Marydale groundwater in some boreholes is of predominantly NaCl type and the nitrate concentration of 19-32 mg/L as N exceeds ideal limits for drinking water of 6mg/L as N . Two soil materials were sampled at different depths from a red sand overlying calcrete (Plooysburg form, Family Py1000). The incubation experiments showed denitrification was complete within a period of between 1 and 6 weeks depending on the carbon substrate and C:N used. Higher rates of nitrate removal were achieved where greater C:N was used. Readily degradable carbon substrates e.g. glucose showed rapid denitrification, while sawdust, a slowly degradable substrate, effected slower denitrification, hence it was concluded that intermediately degradable carbon substrates e.g. wheat straw may prove more suitable. Use of shallower soil material containing initially higher nitrate levels resulted in better denitrification rates, however, both soil materials effected denitrification.. Heterotrophic plate counts increased with time, this presence and growth of heterotrophic bacteria confirmed that conditions were optimum for growth and denitrification and that inoculation with bacteria is not a requirement for in situ denitrification. Dissolved organic carbon (DOC) concentration could be directly correlated to the initial input of carbon substrate as soil and groundwater lacked organic material. Results showed that reaction products such as acetate and nitrite, and basic cation concentrations were elevated in the supernatant solution in preliminary experiments. This was interpreted to be attributed to incomplete oxidation of organic material and excess soluble and available carbon for reaction. Cation concentrations were interpreted to have resulted from a decrease in pH brought on by organic acids produced during denitrification. The method used showed specificity, as the only parameters affected by the denitrification experiment were DOC, alkalinity, nitrite, nitrate, and the heterotrophic plate count. The DOC and HPC did not comply with acceptable levels for drinking water. Removal of HPC by boiling or chlorinating is required to ensure that the resultant water composition is of potable quality. For further research with slowly degradable carbon sources it is recommended that a C:N ratio of more than 12 should be employed, and monitoring should focus on soluble carbon nitrate, nitrite, and heterotrophic plate count. The study confirmed that denitrification of this groundwater with a range of carbon sources is possible within a short period of anaerobic contact with local soil material. With sufficient knowledge of the characteristics of the soil and groundwater in the area, establishment of a working in situ denitrification plant is probably feasible.