72 Groundwater Quality: Remediation and Protection (Proceedings of the GQ'98 Conference held at Tubingen, Germany, September 1998). IAHS Publ. no. 250, 1998.

Groundwater monitoring of the areas contaminated as a result of the accident

SERGEI DZHEPO, ALEXANDER SKAL'SKIJ & DMITRI BUGAI Institute of Geological Sciences, Gonchara Str.55-b, Kiev 252054,

Abstract The paper discusses methodological problems related to groundwater monitoring in the Chernobyl exclusion zone. Data collected as part of a groundwater monitoring programme at Chernobyl have been of rather low reliability until recently due to the following shortcomings: contamination of bore holes during well installation, lack of monitoring well purging before sampling, and inappropriate monitoring well design. Based on the experience acquired, an advanced design of observation well has been developed, and changes have been introduced to the sampling protocol.

INTRODUCTION

Problems of groundwater contamination by radioactivity are most important in the 30 km radius "exclusion" (evacuation) zone, surrounding the Chernobyl nuclear power plant (CNPP). The major Chernobyl long-lived radioactive contaminant of concern from the viewpoint of hydrogeological migration is 90Sr. Large-scale groundwater contamination by radionuclides became evident in 1989-1991. By 1997 the upper portion of the unconfined aquifer in the central part of the Chernobyl exclusion zone was contaminated throughout by 90Sr in concentrations exceeding the Ukrainian drinking water standard of 2 Bq l"1. The most serious sources of groundwater contamination are the "sarcophagus", dumps and the cooling pond, where 90Sr concentrations in groundwater exceed drinking water standards by a factor of up to 103-104 (Dzhepo et al., 1996). Unfortunately, data collected as part of the groundwater monitoring programme in the 30 km zone has until recently been of rather low reliability. We briefly review some of the most common problems related to groundwater monitoring.

METHODOLOGICAL PROBLEMS OF GROUNDWATER MONITORING

Contamination of boreholes during installation of monitoring wells has been a common problem. The typical time dynamics of radionuclide concentrations in samples from such wells is shown in Table 1. The maximal radionuclide activities in groundwater samples were observed immediately after well installation. Some relatively geochemically immobile radionuclides (e.g. 95Zr and 144Ce) were also detected in the groundwater, which is indicative radioactive contamination with fuel "hot" particles introduced to the borehole from the ground surface. Self- decontamination of boreholes due to wash-out of water-soluble chemical forms of Groundwater monitoring of the areas contaminated as a result of the Chernobyl accident 73

Table 1 Radiological sampling data for groundwater monitoring well no. 64, Bq l"1.

Sampling date 144Ce m~ Cs 137^ Cs 95Zr 95Nb l06Ru 90Sr ~ 16 April 1987 11.11 1.70 4.81 1.48 3.33 15.19 21.9 4 May 1988 5.56 2.00 7.78 3.70 0.29 7 July 1988 - 0.48 0.78 0.89 0.44 24 August 1989 0.24 0.05 0.11 0.04 0.15 11 April 1990 - 0.04 - 0.30 10 January 1991 - 0.06 0.34 1.78 radionuclides took 1.5-2 years. Lack of monitoring well purging before sampling was another shortcoming of the sampling protocol. Well purging before sampling (i.e. pumping from the well of a minimum of two to three borehole volumes of water) is a well known fundamental groundwater sampling rule. The aim of this procedure is to replace "stagnant" water inside the well with "fresh" groundwater from the aquifer. Unfortunately, the monitoring staff only started to systematically purge wells before sampling at the end of 1992. Purging of monitoring wells caused an immediate increase in ^Sr activity in groundwater samples from monitoring wells, thus showing the importance of the well purging procedure (Fig. 1(a)). Corrosion of the steel monitoring well casing may cause artificial changes of the natural hydro-chemical and radiological groundwater regime. The wells of the groundwater monitoring network in the 30 km zone usually have steel casings, which are subject to corrosion. The studies of Dzhepo et al. (1994) have shown that iron- ion concentrations in steel monitoring wells are 2-3 times higher compared to the adjacent natural groundwater system, and that corrosion of well casings may lead to a significant decrease in 90Sr concentration in solution due to radionuclide sorption (co-precipitation) on iron hydrous-oxides in the oxidized water column of the well. In addition, the monitoring wells were often of an inappropriate design. Most monitoring wells installed at the CNPP site in 1989-1990 have 12-m-long screened sections. Therefore sampling from these wells produces vertically-averaged ground­ water samples. Interpretation of such sampling data is difficult. By contrast, auger sampling usually shows a distinct vertical distribution of radioactive contaminants in the aquifer, the upper portion of the aquifer being most contaminated (Fig. 1(b)).

CURRENT GROUNDWATER MONITORING PRACTICES

At the present time groundwater observations in the exclusion zone are carried out at about 120 wells. The wells are rather irregularly spaced within the study area. The location of many wells does not account for specific hydrogeological and radiological conditions. It is recognized that the number of observation wells is insufficient, and that the monitoring well network requires extension and modification (e.g. Dzhepo et al., 1996). In particular, a decision has been made to construct new monitoring wells out of poly-vinylcloride (PVC) instead of steel. Also, the new monitoring wells will have shorter (1-2 m long) screened sections. It is expected that in the near future about 20 new monitoring wells will be constructed each year to extend (replace) the groundwater monitoring network in the Chernobyl exclusion zone. 74 Sergei Dzhepo et al.

100Q

100

ra c o

0.1 1 10 100 1000 Sr-90 concentration before purging, Bq I"1

(b) auger-sampling well K-1/1 s J \ ground surface

groundwater level

Explanation: - Sr-90 concentration in groundwater, Bql"1 Fig. 1 (a) Change of '"Sr concentration as a result of purging monitoring wells in the Chernobyl exclusion zone in autumn 1992-spring 1993. (b) Distribution of '"Sr in groundwater near monitoring well no. K-1/1 in the Chernobyl exclusion zone in summer 1994.

REFERENCES

Dzhepo, S. P., Bugai, D. A., Goudzenko, V. V., Skal'skij, A. S. & Varzatski, O. A. (1994) On the question of the methodology of groundwater sampling for radionuclide analysis in the Chernobyl exclusion zone (in Russian with English summary). In: Problems of the Chernobyl Exclusion Zone, no.l, 87-92. Naukova Dumka Publishers, Kiev, Ukraine. Dzhepo, S. P., Skal'skij, A. S. & Bugai, D. A. (1996) Problems of groundwater monitoring in the Chernobyl exclusion zone (in Russian with English summary). Geologicheskij Zhurnal 1/2, 107-112.