應用氡-222 為示蹤劑進行含氯有機溶劑污染場址土層中可能殘留區位調查

地表下氯化有機物污染殘留區位之確認是地下水污染整治成效之重要關鍵，因其為重質非水相液體之特性，於地表下之移動受地質因素影響很大，不易由地下水中污染物濃度正確判斷污染物殘留區。地表下自然產生之放射性同位素氡-222已被應用作為地表下有機污染物殘留區調查之示蹤劑，本研究即於一氯化有機物污染場址進行氡之靜態分析及動態分析，作為污染殘留區之調查應用。靜態分析以比較各監測井地下水氡濃度比值，研判可能之污染殘留區，其判定結果與實場調查顯示之高污染區位相符，兩處原始污染發生區及執行多年抽水處理區之氡濃度比值都相對較低，顯示土層中污染物殘留之可能性。動態分析係進行單井注入抽出試驗，比較抽水階段保守性示蹤劑溴離子與氡-222之貫穿曲線，由氡-222之遲滯情形，研判殘留區污染量，或比較不同試驗井之氡-222貫穿曲線，研判各試驗井區域污染物相對殘留量。試驗結果顯示貫穿曲線之形態受地層透水差異性及試驗井篩區間影響，氡與溴離子貫穿曲線關係受試驗井構造影響，如井篩長、濾料層，未來進行氡之動態分析時應先瞭解試驗區之地層條件，並以專門設計之試驗井進行單井注入抽出試驗，才能獲得預期想完成的目標。

關鍵字

氡-222 ；示蹤劑；單井注入抽出試驗；重質非水相液體；貫穿曲線

並列摘要

1. Introduction It is a difficult task to verify the vertical and spatial distribution of the source zone or the residual region under the surface of a chlorinated contamination site, because of the properties of dense non-aqueous phase liquids (DNAPLs) and the unpredictable geological conditions. Since they have specific gravities greater than water, DNAPLs are able to penetrate through saturated zones until they encounter an aquitard, and then perch on the top of this to form a pool. The paths DNAPLs travel depend on the geological texture, and it is not unusual to find that their distribution differs from the groundwater flow pattern due to geological heterogeneity. It is thus not reliable to verify the source zone by only depending on the DNAPLs concentration distribution in groundwater, although finding this and the residual zone of DNAPLs is critical for the remediation of a contaminated site. Based on numerous NAPL-contaminated site investigations, radon-222 (hereafter referred to as radon) that is naturally present in groundwater can be used as a tracer to locate contaminated sources and quantify contaminants due to the preferential affinity of radon to NAPL, which results in radon deficiencies in groundwater in the region where residual contaminants exist. Radon deficiencies are highly sensitive to small amounts of saturated residual NAPL. Previous research reported that radon concentrations in contaminated source regions reduce to the range of 30 ~ 60% of the background radon concentration in local groundwater. There are two ways to apply radon as a tracer for the investigation of contaminant residual region, the static method and dynamic method. For the static approach, the radon concentrations in the groundwater are normalized to the radon ratio of the maximum concentration, C/C_(max). The radon ratio contours are able to suggest the potential contaminant residual regions in which there are lower radon ratios. In contrast, a single well push and pull test (SWPPT) is used for the dynamic approach. A solution containing a conservative tracer is injected from an injection well, and once the injection is complete the extraction is carried out immediately, and during the extraction stage groundwater is sampled at specific time intervals to analyze the bromide and radon concentrations. The breakthrough curves of bromide and radon are then plotted, and by comparing these the retardation coefficient of radon can be obtained. This retardation refers to the relative saturation of DNAPLs in the saturated zone. This study has two main purposes, as follows. Firstly, using the static method to investigate the potential residual region of contaminants; and secondly, evaluating the performance of the dynamic approach in a chlorinated solvent contaminated site. 2. Material and Methods For the static method, 29 monitoring wells were sampled using the micro purge water method for radon measurement. Every sampling started with flushing the stagnant water in the monitoring well and in the screen zone. Before the samples were taken for radon measurements, the wells were purged until the pH, temperature, and conductivity became stable. A peristaltic pump was used for continuous groundwater sampling at an average flow rate of 0.3 L min^(-1). A 40-ml glass vial with a TEFLON lined cap was used for collecting ground-water samples. After collecting a sample, the sample vial was inverted to check for air bubbles. If any bubbles were present, the sample was discarded and the sampling procedure repeated. The date and time of sampling was recorded and the sample stored in a cooler. The time between sampling and analyses should be less than four days for radon. Radon was analyzed using a liquid scintillation counter by setting the energy window in the range from 500 to 1,000 and employing OPTI FLUOR for the scintillation liquid. 15 mL of water samples were moved into 23-mL counting vials containing 5 mL of the scintillation liquid, which were then placed on an analytical balance to obtain the actual amount of the water samples in the vials. The vials were vigorously shaken by hand, mixing the contents and enabling the radon contained in the water samples to disperse into the scintillation liquid. Finally, before counting, the vials were placed in the dark for a minimum of 3 h until the radon reached secular equilibrium with its short half-life progenies. Duplicate counting samples were prepared for each water sample. For the dynamic method, six wells of SWPPT were used, and ~250 L of test solution was injected for each SWPPT test. The test solution consisted of tap water containing ~100 mg/L bromide, prepared from potassium bromide to serve as a conservative tracer. Test solutions were injected at a flow rate of 1.5 L/ min using a peristaltic pump and the test solution/groundwater mixture was extracted immediately once the injection was complete, using the same flow rate as that in the injection stage. Approximately 500 L of injected solution and groundwater was removed from the well. Water samples were collected for bromide and radon analyses at 20-min time intervals. SWPPT data analysis was performed using normalized bromide and radon concentrations. The normalized bromide concentration is defined as C* = 1 - C/C_0, where C is the measured bromide concentration in a sample and C_0 is the bromide concentration in the injected test solution (~100 mg/L). This calculation is performed to facilitate the comparison of bromide and radon breakthrough curves. The normalized radon concentration is defined as C* = C_w/C_b, where C_w is the measured radon concentration and C_b is the background radon concentration. For each SWPPT, the pull phase normalized radon and bromide concentrations were plotted as a function of dimensionless time V_e/V_i, where V_e is the volume of solution extracted at the time a water sample was obtained, and V_i is the total volume of solution injected. 3. Results and Discussion The radon ratio contour based on the radon concentration measurements from 30 monitoring wells indicated three potential contaminants residual regions in the investigated site. These results were consistent with the outcomes of source zone investigations carried out in previous projects. Two of the three potential regions are in the hot spots with the highest concentrations of tetrachloroethylene, trichloroethylene, and 1, 1, 2-trichloroethane in groundwater. The remaining one is located in the region in which a pump and treat system (P & T) had operated for about six years. The operation of pump drew the plume of contamination toward to the P & T for treatment, resulting in the enrichment of contaminants in the subsurface vicinity of the system. The low permeability strata were able to absorb the contaminants from the groundwater, thus producing residual regions. Of the 30 monitoring wells, 18 wells' radon ratios were in the range of 0.5 ~ 0.7, and these results are consistent with the fact that high concentration plumes of cis-1, 2-dichloroethylene and vinyl chloride have extended all over the site for more than a decade. The breakthrough curves of bromide of the SWPPT wells showed some notable differences, and it is suggested that these were due to the various geological settings in which the well screens are located. The wells TW08, TW09, and EPB-MW6D, which had breakthrough curves that were similar to the typical results for a homogeneous sand column test, are those for which the screens were set in strata with mostly fine sand and were also less heterogeneous. The well screen of TW06 was located in a silty clay layer that made the time needed to reach the breakthrough point shorter than seen with other wells. Compared with the bromide breakthrough curve, the radon did not show any notable retardation, and there may be a number of reasons for this, as follows. Firstly, since the test wells existed before the current study was carried out, they may not be located in the contaminant residual regions. Secondly, the background radon concentration of each test well was not analyzed prior to SWPPT, and this may have made the normalized concentrations for the breakthrough curves too high. The virtual background radon concentrations are supposed to be greater than the radon concentrations in the final stage of the extraction phase, and the forme are referred to as the background radon concentrations when plotting breakthrough curves. Thirdly, the screens of the test wells were too long, and this resulted in the limitation of extending SWPPT to only within 10 cm away from the borehole filling boundary. The portion of strata that the SWPPT solution covered was thus too small to present the retardation of radon movement. 4. Conclusions (1) The radon ratio contour derived from the static method of radon measurements was able to indicate the potential residual region of contaminants. (2) The geological settings influence the breakthrough curve pattern, and it is thus necessary to better understand the geological texture prior to the design of SWPPT well installation. (3) The background radon concentration of each SWPPT well had has to be analyzed prior to carrying out the SWPPT in order to obtain the virtual breakthrough curve of radon. (4) The shorter the length of SWPPT well screens the larger the coverage of SWPPT injection solution. Shortening the screen length from 6 m to 1.5 m extends the influence radium of the solution from 10 cm away from borehole filling boundary to 45 cm away. (5) The voids in the borehole that exist between the pieces of gravel that fill it provide spaces to store the solution thus reducing the amount of solution pushing into the strata and interfering with in the breakthrough curves.

並列關鍵字

Radon-222 ； tracer ； single well push pull test ； dense non-aqueous phase liquids ； breakthrough curve

國際替代計量

應用氡-222 為示蹤劑進行含氯有機溶劑污染場址土層中可能殘留區位調查

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