Abstract:

The accurate monitoring of groundwater quality depends on the construction of high-quality groundwater monitoring wells, and the suitability and stability of well construction materials are key factors for the construction quality of groundwater monitoring wells. To investigate the stability and metal element leaching risk of well construction materials of groundwater monitoring wells under long-term corrosion, PVC-U pipes and stainless steel pipes were used as well pipes to design two types of single-pipe and single-layer groundwater simulation monitoring wells with the addition of filling materials (bentonite and quartz sand) and stainless steel mesh, respectively. Acidified groundwater (pH 4.0), originally collected groundwater (pH 6.0), and alkaline groundwater (pH 8.5) were added to two simulated monitoring wells to maintain an appropriate water elevation. Water samples were collected regularly (1, 7, 14, 30, 60, 90, 120, and 180 days) and replenished with groundwater. The water in the simulated monitoring wells could maintain a certain degree of exchange, and the cumulative release amounts and release sources of metal elements in the well pipe materials and filling materials during different periods were studied. The single-factor pollution index method was used to evaluate the leaching risks of metal elements. The results showed that the release amounts and leaching concentrations of Fe and Ba from the well construction materials of the simulated monitoring well were high, and followed by Mn and Zn, their maximum values were 1.07‒3.36 mg and 29.9‒90.4 µg∙L⁻¹, respectively. The maximum release amounts and leaching concentrations of Al, Ni, Pb and Mo were higher than those of Ti, V and Cr, and the values of the former were 236‒697 µg and 6.69‒17.9 µg∙L⁻¹, respectively; and the values of the latter were 32.6‒75.9 µg and 1.09‒1.98 µg∙L⁻¹, respectively. The amounts of Cd and Cu released were low, while the amounts of Ag, Be, Hg, Se, and Sn released were too low to be detected. The concentrations of most metals in the groundwater of the simulated monitoring well increased with increasing soaking time. In acidic groundwater, the concentrations of Al, Ba, Cd, Cr, Fe, Mn, Mo, Ni and Pb tended to equilibrium after 120 days, and in alkaline groundwater, the concentrations of Ba, Cd, Mn, Mo and Zn tended to equilibrium after 90 days, and Al, Cr, Fe, Ni and Pb tended to equilibrium after 120 days. The released amounts and leaching concentrations of metal elements in groundwater (pH 4.0) were significantly higher than those at pH 6.0 and pH 8.5 groundwater, and the release degree of metal elements in well construction materials increased with decreasing groundwater pH value. The released amounts and leaching concentrations of the majority of the metal elements in the stainless-steel pipe groundwater were higher than those in PVC pipe groundwater. The leaching of Al, Ba, Cd, Mn, Pb and Zn might be mainly controlled by the release of the filling materials, while the leaching of Cr, Cu and Mo was mainly released by the filling materials, followed by the release of stainless-steel materials. The leaching of Fe, Ni, Ti, and V was mainly controlled by the release of stainless steel materials. The leaching risk of metal elements in the well construction materials was mainly at the level of no risk and slight risk, and the leaching risk was generally low, which did not affect groundwater quality. This study provides a basis for selecting of suitable construction materials.

Key words: groundwater monitoring wells, well construction materials, metal elements, release characteristics, leaching risk

摘要：

地下水精准监测有赖于高质量的地下水监测井建设，建井材料稳定性是地下水监测井建井质量的关键因素。分别以PVC-U管和不锈钢管作为井管设计两种单管单层式地下水模拟监测井，分别加入pH 4.0、6.0和8.5地下水使水面达到合适高度，定期（1、7、14、30、60、90、120、180 d）采集水样和补充地下水保持水体一定程度循环，考察长期水浸状态下建井材料中金属元素时间累积释放量及其来源，采用单因子污染指标法评估金属元素溶出风险。结果表明，模拟监测井中Fe、Ba释放量较高，其次为Mn、Zn，它们最大释放量和相应溶出质量浓度分别为1.07-3.36 mg和29.9-90.4 µg∙L⁻¹；Al、Ni、Pb、Mo相比Ti、V、Cr较高，释放量和溶出质量浓度最大值分别为236-697 µg、6.69-17.9 µg∙L⁻¹和32.6-75.9 µg、1.09-1.98 µg∙L⁻¹；Cd和Cu释放较低，而Ag、Be、Hg、Se和Sn等元素未检测出。监测井水中大多数金属元素质量浓度随着时间增长而升高，并于120 d后金属元素质量浓度趋向平衡。建井材料中金属元素释放强度随地下水酸性增强而提高，pH 4.0水中金属元素释放量和溶出浓度显著高于pH 6.0和8.5时。不锈钢管建井条件下地下水中多数金属元素释放量和溶出浓度高于PVC管建井条件下值。Al、Ba、Cd、Mn、Pb和Zn溶出可能主要由填料释放控制；Cr、Cu和Mo溶出主要是由填料释放，其次受不锈钢材料释放的影响；而Fe、Ni、Ti和V溶出主要由不锈钢材料释放控制。建井材料金属元素的溶出风险总体处于无风险和轻微风险水平，总体溶出风险较低而不影响地下水水质。该研究可为合理选择建井材料提供数据基础。

关键词: 地下水监测井, 建井材料, 金属元素, 释放特征, 溶出风险