Abstract:

The Nemerow index method is commonly used for environmental quality assessment. However, the weights used in the evaluation process are not considered in the traditional Nemerow index method. Consequently, the maximum value is overemphasized among the pollution factors, which can lead to evaluation results that deviate from the reality. Currently, the research scale of volcanic rock underlying the compound aquifer system in the Qiongbei area of Hainan Province is mostly focused on a range of 1꞉200000‒1꞉100000, while medium- or large-scale studies have rarely been reported. In this study, eight groundwater samples were collected from designated locations and their water pollution characteristics and causes were analyzed. Fe, As, Hg, Cd, and Pb were selected as typical heavy metal pollution factors, and environmental quality was comprehensively evaluated using four types of Nemerow index methods, both before and after modification. The results indicated that the three modified Nemerow index methods compensate for the shortcomings of the traditional method. Specifically, the index values of the eight samples calculated using the modified Nemerow index method were 0.66, 0.06, 1.11, 1.39, 0.72, 0.22, 0.45, and 0.22, which correspond to the standards of groundwater quality categories Ⅱ, Ⅰ, Ⅳ, Ⅳ, Ⅲ, Ⅰ, Ⅰ, and Ⅰ, respectively. Water quality was mainly affected by Fe with a weight ratio of 85%, followed by Cd (10%) and Pb (10%), and the least effect was observed for As (5%) and Hg (5%). The modified Nemerow index method fully considers the weight and scoring mean value of the maximum pollution factor, and the correction of the maximum pollution factor compensates for the defect of overemphasizing the maximum value and ignoring the contribution value of the factor with a lower concentration in the traditional Nemerow index method. This method ensures that the evaluation results are consistent with objective reality. The water quality of the second confined water in the study area was relatively good, whereas Fe, As, and other factors exceeded the standard with a scattered distribution. The main sources of Fe were the upper Beihai Formation (Qp2b) and volcanic rock weathering residue (slope) deposits. Direct recharge after precipitation leaching and recharge through seepage after entering the groundwater aquifer are considered to be the main means of Fe transportation. The research area is located in a concentrated recharge area where basalt is exposed at the surface, and the unconfined aquifer has poor pollution-prevention capabilities. Large-scale and persistent sources of pollution inevitably lead to regional groundwater contamination. Therefore, great attention should be paid to groundwater protection in the study area and preventive measures should be taken in advance.

Key words: Nemerow index method, groundwater quality assessment, large-scale map, heavy metal, environmental quality

摘要：

传统的内梅罗指数法在评价过程中未考虑权重，过分强调污染因子中的最大值，从而导致评价结果脱离实际。同时，针对火山岩下伏复式含水层系统的研究，海南省琼北地区比例尺多集中在1꞉200000－1꞉100000之间，中大比例尺的研究工作几乎处于空白。以1꞉50000白莲市幅第2承压含水层（组）（海口组一段）水文地质条件调查研究为基础，对8个点位地下水样品进行采集并分析其水质污染特征和污染成因;运用修正前、后4种内梅罗指数法，选取Fe、As、Hg、Cd、Pb等5个重金属污染因子，对其环境质量进行综合评价。结果表明，修正后的3种内梅罗指数法弥补了传统内梅罗指数法的不足。水质主要受Fe的影响最大，占权重的85%;其次是Cd和Pb，占10%;As和Hg影响最小，占5%。该方法充分考虑污染因子最大值的权重和评分均值，可以对最大污染因子数值进行修正，弥补过于突出最大值，而忽略浓度较小因子贡献值的缺陷，评价结果更符合客观实际。研究区第2承压水水质整体较好，局部有Fe、As等因子超标，但呈零星点状分布;研究区处于集中补给区，玄武岩出露地表，潜水含水层防污性能差，若污染源量大，且有持续性，必将引发区域性地下水污染，应引起足够重视，防患于未然。

关键词: 内梅罗指数法, 地下水水质评价, 大比例尺, 重金属, 环境质量