Hydrogeological condition at the water works of Elverum kommune

Abstract

Norwegian rivers and lakes can provide abundant amount of fresh water being more than 60% of these surface water sources in high ecological status. About 15 % of water consumption in Norway is using groundwater as a source. Grindalsmoen water work located in Elverum is one of the biggest water work of Norway which uses a ground water as raw source of water for municipal water supply. The water work is supplying fresh water for the population 17,000 by making pumping through 4 extraction wells. The raw water consists of iron and manganese as impurities in natural condition which are in dissolved condition. These impurities are separated from raw water through Vyredox method where air is inject to a ground water through 9- satellite wells located around the main pumping well. Glacio-fluvial and alluvial deposits are major deposits from few hundreds meter to kilometres from bank of river in both direction. This type of geological condition favours extraction of groundwater, and has possibility of high amount of groundwater recharge compared to the till deposits located at outer region of river. The objective of this study was to improve the understanding of the geological Condition and groundwater flow at Grindalmoen. Based upon these information we tried to understand the response of aquifer system during annual fluctuation on recharge and continuous pumping on water work area. The preliminary information like Quaternary maps, annual precipitation, position of bedrock and stable water position was obtained from different sources like NVE, Granada data Base etc. The annual recharge was taken as 283.4 mm/year where 1/3 of recharge is assumed to account for autumn recharge and 2/3 of total recharge accounts for snow melt. To improve the understanding of the detailed geological condition, a field course was conducted during the summer of 2017 within collaboration between NMBU and Oslo University. During the field course, geophysical explorations, drilling and infiltration tests were conducted to understand the geological situation around Grindalmoen water work area. The task was done into two methods to understand the soil surface property and sub-surface conditions. The test like infiltration test, slug test and grain size distribution were conducted to determine the hydraulic conductivity of water work area. The average hydraulic conductivity of surface soil was found to be 0.000634 m/sec. This value represents to hydraulic conductive of fine soil particle and from grain size distribution result also we got a similar type of soil property in top surface. The geophysical investigation was performed through electrical resistivity (ERT) survey, GPR survey and seismic survey. From ERT results we obtain heterogeneous soil property with fine sand on top surface and saturated coarser soil underneath the top surface which is controlling the flow around a water work area. The GPR survey showed a clear location of ground water within 5m meters from the top surface and its flow direction was towards the river with mild gradient. The GPR survey made clear visualization of dense surface at almost 30 meter around waterworks area indicating a confining layer above a more conductive aquifer below. The seismic survey gave an indication of location of ground water table. The numerical model was developed using a graphical user interface called Modelmuse together with the groundwater flow model Modflow-2005 to represent the geometry, including surface topography, and hydrogeological conditions of the aquifer. Information on hydraulic conductivity, recharge rate, pumping rate and positon of bedrock was introduced in the model to make the numerical model similar to real condition. The model was run for both steady state and transient state making several stress period similar to annual recharge fluctuation for 15 years. The observation was made in three different location of catchment area where were we observe clear fluctuations in head values during annual fluctuation of recharge. The pumping test was made with the same value of pumping rate i.e. 60 litres per second that Grindalmoen water work makes and with rate of 600 litres per second to observe the drawdown in the well area. The modelling result gave the draw down head values up to 1.75m in water work area which is quite near the observed Drawdown value which was up to 2 meters during pumping at rate of 60 litres per second in Main well 4 of water work area. Further we observe that the drawdown curve did not reached to the constant head which was considered as supply of continuous water to system. But during pumping at rate of 600Litres per second drawdown was only 3.6 meter although pumping rate was 10 times higher than the normal pumping rate. The sensitive analysis was made by varying the hydraulic conductive and recharge in catchment area. The result indicates that the aquifer is more sensitive toward hydraulic conductive compared to recharge. From the detailed study we came to know that the top surface of water work is dominated by fine sand and medium sand with lower conductivity and the subsurface consists of completely saturated coarser sand. The ground water is located at shallow depth of 5meters near water work and flow direction is almost perpendicular in norther area of water work but slowly follows in parallel direction as well move downward to water work area. The numerical modelling showed that the aquifers make change in head value in accordance with fluctuation in recharge periods. The drawdown curve made due pumping at rate of 60 litres per second doesn’t meet the constant and was similar with actual field condition. So can make assumption that there is little influence on aquifer due to drawdown of head during pumping at rate of 60 litres per second. The entire study including modelling was done based on field study and information collected from secondary sources. Although the geophysical measurements and drillings provide useful information about the heterogeneity and hydraulic conductivity of the area, it is still limited for a very detailed understanding of the hydrogeology of the water works area. The information regarding hydraulic conductivity of soil was just determined for top soil. But to obtain much more accurate results several no of geophysical study need to be made so collect this sub-surface information. The bedrock was located up to depth 30 meter near water work and was even exposed in western part of catchment area. This steep gradient of the underlying bedrock is difficult to represent with the rectangular grid used in Modflow, alternatively a more sophisticated software using finite element or much finer grid resolution in Modlfow is required for a better representation of the real aquifer geometry. The results obtained from this study could be used to make further investigation of the water work area. The flow pattern of ground water could be used to conduct more geophysical investigate in that area