From remediation supervision to long-term surveillance – Ground and surface water monitoring at a former uranium processing site Annia Greif, Sven Eulenberger, Peter Schmidt, Michael Paul Wismut GmbH, Jagdschänkenstraße 29, 09117 Chemnitz, Germany, [email protected] Abstract Starting in 1990, the clean-up of the former Crossen uranium mill near Zwickau in Germany has been a focal point in the Wismut remediation program which is dedicated to the legacy of large scale uranium mining and milling in East Germany. The discharge of treated water, but also the still remaining release of contaminated seepage into aquifers and surface streams require the monitoring of the water-borne en- vironmental impact. Key contaminants include uranium, arsenic and radium, accom- panied by molybdenum, sulphate, chloride, hydrogen carbonate and sodium. The current remediation monitoring is based on radiation protection licences and permits issued under water law. With the transition towards post-remedial and long- term activities after 2020 the monitoring requirements will substantially change regard- ing key function and scope. The presented paper describes the existing water monitor- ing system and key findings of the previous monitoring period. Finally, site conditions, constraints and remaining uncertainties for post-remedial monitoring will be discussed. Keywords: ore processing, tailings pond, uranium, arsenic, radium, monitoring, groundwater, surface water Introduction floodplains of the Zwickauer Mulde river to The second largest tailings management the Helmsdorf TMF (terminated in 2018) facility (TMF) of the former East German and the rehabilitation of the footprint and uranium mining industry (former SDAG reuse as a renaturalized floodplain; Wismut) is situated at Crossen and - and the dry stabilization of the TMF itself Helmsdorf near the city Zwickau, Saxony. on 200 hectares (to be finished in 2021). Ore processing and the deposition of The technology of in situ decommissioning residues occurred on both sides of the includes the expelling of pond water, interim river Zwickauer Mulde. First, three smaller covering of the tailings surface, dewatering of tailings ponds (TP) were piled up from 1950 the fine tailings, re-contouring of dams/ponds to 1958: Crossen (0.3 million tons), Dänkritz and final covering including re-vegetation I (6.6 million tons) and Dänkritz II (1.2 (Neudert & Barnekow 2006). The water million tons). After the depletion of storage management strategy involves the treatment capacity TMF Helmsdorf (49 million tons) of contaminated pond, ground- and seepage was started in 1958 and worked until 1989. water (Laubrich et al. 2018). Site remediation included The discharge of treated water, but also - the decommissioning of the processing the still remaining release of contaminated plant including area clean-up (finished seepage into aquifers and surface waters in 2008), other facilities on the left side of require the monitoring of the water-borne the Mulde were already dismantled before environmental impact. 2001; With the transition towards the - the relocation of the huge Crossen waste implementation of long-term tasks, the pile (3.3 million cubic meters) including requirements regarding function and scope the underlying older tailings from the of the environmental monitoring will change. 660 Wolkersdorfer, Ch.; Khayrulina, E.; Polyakova, S.; Bogush, A. (Editors) IMWA 2019 “Mine Water: Technological and Ecological Challenges” Geological-hydrogeological site the tailings facility. Morphologically, these characterization structures are visible through valley cuts, The Crossen-Helmsdorf site is located in such as the Oberrothenbacher, Wüster- the western part of a permian molasse basin Grund, Niederhohndorfer, Zwischengrund between the Erzgebirge in the south and the and Trischgrund valleys. These structures Granulitgebirge in the north, which is based extend into the area of the Zwickauer Mulde on old Paleozoic and Precambrian basement. floodplain and pave the way for contaminated The discontinuously outcropping sediments groundwater (Möckel & Neudert 2004). of the Upper Lower Carboniferous up to the The completely remediated Dänkritz I TP Oberrotliegend II are continental sediment is located in an area of tertiary loose sediment deposits of the eroded Variscan orogen. deposits, having been subject to intensive The underground of the Helmsdorf TMF is sand and gravel extraction in earlier times. determined by rocks of the Mülsen formation Tailings were deposited inside an exploited of the Oberrotliegend, which is intensively gravel pit. The clastic materials represent tectonically stressed in the vicinity of a pore aquifer of good water permeability Helmsdorf TMF. The characteristic tectonic along a SW-NE directed palaeo-channel. The structural pattern combines structures which relief of the underlying Rotliegend surface strike in ENE-WSW, NW-SE, N-S as well as determines the groundwater flow direction NNW-SSE direction (fig 1). within the Tertiary sediments towards the Based on intensive drilling work from Zinnborn/ Zinnbach creek. Due to a high 1998 to 2003 and the gradual commissioning uranium contamination of groundwater of groundwater monitoring wells during downstream of the Dänkritz I TP, two the rehabilitation period, it was recognized extraction wells went into operation in that all ENE-WSW striking fault zones on 2002/2003, whose water is treated together the eastern flank of the Helmsdorf TMF are with seepage water of the TMF Helmsdorf in involved in groundwater runoff affected by the water treatment plant (WTP) Helmsdorf Figure 1 Overview of the Crossen-Helmsdorf site including the most important objects, water catchments, schematized tectonic fault zones and points of key monitoring network 2018 Wolkersdorfer, Ch.; Khayrulina, E.; Polyakova, S.; Bogush, A. (Editors) 661 IMWA 2019 “Mine Water: Technological and Ecological Challenges” (Laubrich et al. 2018). The Dänkritz II TP, Configuration and modification of also situated inside of an exploited gravel pit, key monitoring network is not subject to the obligation of Wismut A key measuring network is composed of GmbH. The remediation will start in the near measuring points, which are needed for a future. sufficient characterization of the hydraulic The other objects (Crossen waste dump, and hydrochemical system behaviour in time processing plant and further industrial area) during aftercare (Sporbert et al. 2006). are located in the floodplain of the Zwickauer Decisive for the selection of key measuring Mulde. It is characterized by quaternary points is the comprehensive knowledge loose sediments in the subsurface, which are of the geology and hydrogeology of the extensively covered by alluvial loam. respective remediation object and the pathes Basics of monitoring networks de- of influence. All relevant aquifers or water- velopment bearing units influenced by these objects be considered. Based on the groundwater flow Starting in 1990, first measuring networks regime, groundwater inflows and outflows of each compartment (water, air, soil) were are assigned to the object. The monitoring of installed in order to identify pollution the pollutant source (e.g. tailings pond) must sources and to describe their environmental be included in case of in-situ-remediation impacts. After the deduction of remediation (Möckel 2006). needs a selection of substantial (key) With the help of key monitoring measuring points for the water path are networks, the fundamental, spatially defined as basic monitoring network which substantial influences of groundwater by is legally based on the specifications of the objects of Wismut GmbH are to be tracked REI Bergbau (1997). These points concisely in the long term and to be judged in terms characterize the site and serve for long-term of their dispersion dynamics as well as their oberservation. By setting limit values, water trend behavior. and radiation protection approvals determine In the course of the transition from the requirements for self-control with regard remediation to long-term monitoring, the to parameters and sampling frequency. water monitoring serves for: Aspects of monitoring tasks are summarized – Evidence of hydraulic and geochemical in table 1. stability after completion of remediation, The remediation procedure requires – Exclusion of a trend reversal of already the observation of each individual object, decreasing or stable concentration values the compliance with limits and a flexible in groundwater, adaptation of monitoring elements to the – Prevention of health problems for the clean-up progress. In expectation of the end population from contaminants spreaded of the physical remediation work the complex through the groundwater path, which monitoring network will be reviewed and could possibly lead to restrictions on its adapted to the needs of the post-remedial use for human consumption. or long-term phase. In conjunction with During the initial phase of the rehabilitation resulting (long) time series the long-term project until 2003, the groundwater monitoring network should reflect the success surveillance network at the Crossen/ of remediation activities (Kreyßig et al. 2008). Helmsdorf site embraced more than Table 1 Aspects of monitoring tasks Tasks Classification of monitoring activities Monitoring tasks: screening, remediation, long-term Temporal tasks: before, during, past remediation Object tasks: background, source, path, effect Permit tasks: basis, remediation, after care With respect to extent: