George Nyakno Jimmy
Department of Physics Geophysics Research Group, Akwa Ibom State University, PMB 1162, Mkpat Enin, Uyo, Nigeria.
Appl Water Sci. 2021;11(7):123. doi: 10.1007/s13201-021-01437-x. Epub 2021 Jun 25.
AVI (Aquifer vulnerability index), GOD (groundwater occurrence, overlying lithology and depth to the aquifer), GLSI (geo-electric layer susceptibility indexing) and S (longitudinal unit conductance) models were used to assess economically exploitable groundwater resource in the coastal environment of Akwa Ibom State, southern Nigeria. The models were employed in order to delineate groundwater into its category of vulnerability to contamination sources using the first- and second-order geo-electric indices as well as hydrogeological inputs. Vertical electrical sounding technique employing Schlumberger electrode configuration was carried out in 16 locations, close to logged boreholes with known aquifer core samples. Primary or first-order geo-electric indices (resistivity, thickness and depth) measured were used to determine S. The estimated aquifer hydraulic conductivity, K, calculated from grain size diameter and water resistivity values were used to calculate hydraulic resistance (C) used to estimate AVI. With the indices assigned to geo-electric parameters on the basis of their influences, GOD and FSLI were calculated using appropriate equations. The geologic sequence in the study area consists of geo-electric layers ranging from motley topsoil, argillites (clayey to fine sands) and arenites (medium to gravelly sands). Geo-electric parametric indices of aquifer overlying layers across the survey area were utilized to weigh the vulnerability of the underlying water-bearing resource to the contaminations from surface and near-surface, using vulnerability maps created. Geo-electrically derived model maps reflecting AVI, BOD, FLSI and S were compared to assess their conformity to the degree of predictability of groundwater vulnerability. The AVI model map shows range of values of log C ( -3.46-0.07) generally less than unity and hence indicating high vulnerability. GOD model tomographic map displays a range of 0.1-0.3, indicating that the aquifer with depth range of 20.5 to 113.1 m or mean depth of 72. 3 m is lowly susceptible to surface and near-surface impurities. Again, the FLSI map displays a range of FLSI index of 1.25 to 2.75, alluding that the aquifer underlying the protective layer has a low to moderate vulnerability. The S model has values ranging from 0.013 to 0.991S. As the map indicates, a fractional portion of the aquifer at the western (Ikot Abasi) part of the study area has moderate to good protection (moderate vulnerability) while weak to poor aquifer protection (high vulnerability) has poor protection. The S model in this analysis seems to overstate the degree of susceptibility to contamination than the AVI, GOD and GLSI models. From the models, the categorization of severity of aquifer vulnerability to contaminations is relatively location-dependent and can be assessed through the model tomographic maps generated.
采用AVI（含水层脆弱性指数）、GOD（地下水赋存、上覆岩性及含水层深度）、GLSI（地电层敏感性指数）和S（纵向单位电导率）模型，对尼日利亚南部阿夸伊博姆州沿海环境中具有经济开采价值的地下水资源进行评估。运用这些模型，利用一阶和二阶地电指数以及水文地质输入数据，将地下水划分为不同的污染脆弱性类别。在16个靠近有已知含水层岩芯样本的测井钻孔处，采用施伦贝格尔电极配置进行了垂直电测深技术。测量得到的初级或一阶地电指数（电阻率、厚度和深度）用于确定S。根据粒径直径和水电阻率值计算得到的估计含水层渗透系数K，用于计算估算AVI所需的水力阻力（C）。根据地电参数基于其影响所赋予的指数，使用适当的公式计算GOD和FSLI。研究区域的地质层序由杂色表土、泥质岩（黏土至细砂）和砂质岩（中砂至砾砂）等地电层组成。利用调查区域内含水层上覆层的地电参数指数，通过创建的脆弱性图，来衡量下层含水层资源对地表和近地表污染的脆弱性。将反映AVI、BOD、FLSI和S的地电推导模型图进行比较，以评估它们与地下水脆弱性预测程度的一致性。AVI模型图显示log C值范围为（-3.46 - 0.07），一般小于1，因此表明脆弱性高。GOD模型层析成像图显示范围为0.1 - 0.3，表明深度范围为20.5至113.1米或平均深度为72.3米的含水层对地表和近地表杂质的敏感性较低。同样，FLSI图显示FLSI指数范围为1.25至2.75，暗示保护层下方的含水层具有低至中等的脆弱性。S模型的值范围为0.013至0.991S。如图所示，研究区域西部（伊科特阿巴西）部分含水层的一小部分具有中等至良好的保护（中等脆弱性），而脆弱至较差的含水层保护（高脆弱性）则保护较差。在该分析中，S模型似乎比AVI、GOD和GLSI模型更夸大了对污染的敏感程度。从这些模型来看，含水层对污染的脆弱性严重程度分类相对依赖于位置，可通过生成的模型层析成像图进行评估。
