PREDICTING SHALLOW AQUIFER PARAMETERS AND WATER QUALITY IN OKPARA COAL MINE AREA AND ENVIRONS USING GEOELECTRIC SOUNDING

AUTHOR: UTOM, AHAMEFULA UDUME

DEPARTMENT: GEOLOGICAL SCIENCES

AFFILIATION: NNAMDI AZIKIWE UNIVERSITY, AWKA

A resistivity survey was conducted in the area around the abandoned Okpara coal mine site in an effort to predict shallow aquifer properties and water quality, and to identify potential areas of high concentrations of total dissolved solids (TDS) and sulphate content as an indicator of acid mine drainage. The survey consisted of nineteen vertical electrical soundings (VES) was recorded using Schlumberger configuration. Additionally, water quality in the area was assessed by the analysis of basic physico-chemical parameters (pH, Electrical Conductivity, TDS, Total Hardness, Fe2+, Mn, Ca2+, Mg2+, Na+, K+, HCO3 -, Cl-, NO3 – and Chemical Oxygen Demand (COD)) of 12 water samples: one of the water samples collected from a contact spring running over coal mine spoils (CMS-1) while the remaining eleven water samples were collected from shallow water wells (SWW-2, SWW-3, SWW-4, SWW-5, SWW-6, SWW-7, SWW-8, SWW-9) and boreholes (BH-10, BH-11, BH-12). The water samples were collected in plastic bottles which were pre-cleaned with concentrated hydrochloric acid and distilled water. Water quality analysis in laboratory started about 2 hours after the collection for some of the samples while some of them which were not analyzed for the day were preserved in the refrigerator prior to analysis to exclude microbial activity and unwanted chemical reactions. The general trend among cations showed Ca2+>Mg2+>Na+>K+ while majority of the anions display SO4 2-> NO3->Cl->HCO3- distribution pattern. Results of the sulphate content (SO42-) as an important attribute of water quality, possibly affected by coal mining operations showed up to 33 % of the analyzed water samples (CMS-1 and SWW-8) were above the maximum permissible limit of the Nigerian Industrial Standards (2007) for the sulphate content ranging between 1.87 and 650 mg/L averaging 129.2 mg/L. 33 % of the analyzed water samples (CMS-1, SWW-2, SWW-8, and SWW-9) with potential acid mine drainage contamination ranged from 48.3 to 105.3 mS/m. Impact of acid mine drainage on the analysed water samples was evident but has not seriously degraded the water quality. The VES results revealed a three- to six- layer geoelectric model which include the top soil, coarse-grained sandstone, medium-grained sandstone, fine-grained sandstone, silty sand and coal lenses, shale and coal seam with resistivity values ranging from 218 – 5900 .m, 988 – 10106 .m, 116 – 813 .m, 68 – 200 .m, 28 – 76 .m, 3 – 55 .m and 1326 – 5831 .m respectively. Probable shallow aquifer resistivity, thickness and depth values ranges from 28 – 527 .m, 2.1 – 22.5 m and 3.1 – 28.3 m respectively. The average hydraulic conductivity of 8.96 x 10-2 cm/s and transmissivity ranging from 5.93 x 104 to 6.37 x 104 m2/year estimated from surface resistivity measurements correlated well with the available field data obtained from pumping test analysis (point hydraulic conductivity and transmissivity values were 2.60 x 10-2 cm/s 8.10 x 104 m2/year respectively). Results showed a good positive correlation between aquifer and transverse resistance (R2=0.84). The VES data integrated with physico chemical parameters was used to establish geoelectrically-important hydrogeochemical relationships in an effort to determine the resistivity instruments response to contrasting subsurface characteristics. The data from the study area indicated a positive statistical correlation between pore water conductivity and sulphate content (R2=0.99). A strong correlation (R2=0.99) between theoretical bulk conductivity and bulk conductivity from this work was observed. A linear regression model between earth resistivity and pore water resistivity (R2=0.62: ρw = 0.222ρb + 23.25) suggests the applicability of geoelectric sounding in predicting water quality.

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