VARIABILITY OF FLOW AND CHEMISTRY OF GROUNDWATER IN THE BROAD AREA OF THERMAL EXPERIMENT AT UNDERGROUND RESEARCH CENTER JOSEF, MOKRSKO, CENTRAL BOHEMIA

Assessing the possibility of heat storage in geological environment is the main objective of VITA project. Two years long heating experiment was performed at underground research center Josef in the old gold mine near Mokrsko, Central Bohemia. The experiment is set in the side adit located 2 km far from gallery entrance in the depth 100 m below the surface. Rock environment is formed by middle grained Sázava type granodiorite.

The rock is cyclically heated to 95 ° C and subsequently cooled to the natural temperature of the environment. Thermal changes, rock stress, water flow and chemical changes of groundwater are observed in the closest vicinity of the heater. In broader areas up to 200 m far from experimental site, the geochemistry and flow of groundwater in the mine is measured as a background level. Groundwater levels and rainfall amounts are monitored as an input to the rock environment above the experiment site.

Water inflows to the gallery are in a range from 1,4 mL.min^-1 to 536 mL.min^-1. Maximal observed inflow was up to 840 mL.min^-1. Newly constructed measuring device was developed for small inflows which only drip. This device is recently patented. Seasonal trends were found during the 2 years lasting observations. The weak rise of inflow volume appears in the spring months when snow cover melts. High rise of inflow was observed from June to late August last year 2013 due to extreme rainfalls during the flood. Estimated time of pressure response of pressure head is 2-3 months at 100 m depth. Chemical composition of groundwater varies from Ca-SO₄-HCO₃ to Ca-HCO₃-SO₄ waters with pH values in range between 7.6 to 8.5. Groundwater is slightly mineralized with total dissolved solids between 330 and 410 mg.L^-1.

Consistent trends of the content of cations and anions are present in groundwater. They are seasonal, often with sinusoidal changes. Several inflows were grouped because they act very similarly, the others have its’ own independent flow regime. Recent research does not show any significant influence on the chemistry of groundwater even in the closest distances around the heater. On the other hand massive growth of microbial organisms and newly formed secondary minerals occurs on the walls of the adit.

This work was supported by project FR-TI3/325 funded by Ministry of trade and industry of the Czech Republic.