Artificial Groundwater Recharge in Quaternary Gravel Aquifers in the Foreland of the Alps
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Improvements of Methods of Long Term Prediction of Variations in Groundwater Resources and Regimes Due to Human Activity (Proceedings of the Exeter Symposium, July 1982). IAHS Publ. no. 136. Artificial groundwater recharge in Quaternary gravel aquifers in the foreland of the Alps B, HERRLING Institute of Hydromechanics, University of Karlsruhe, West Germany ABSTRACT The possibility of recharging groundwater with the high-water of streams in valleys of the glacial period which are filled with permeable sediments is numerically investigated. Due to the great distance between the water table and the surface in some of these regions, the use of gravel-pits as storage and seepage basins has been proposed and the consequences downstream in the valley are explored. A time dependent, two- dimensional horizontal finite element model has been employed. In addition, moving boundaries as a result of the sloping lateral edges of the aquifer have been considered within the model. Preliminary results are presented for a case study. INTRODUCTION Numerous glacial valleys in the foreland of the Alps in West Germany consist primarily of Quaternary sand and gravel sediments. In these basin and channel systems the impermeable base not only has very steep slopes at the lateral periphery but often has a slope of more than 1% in the aquifer thalweg. The groundwater flow is mainly charged by the relative high precipitation in these areas and by the inflow from transverse valley aquifers. In some regions the distance between the water table and the surface reaches up to 20 m, whereas at other places the streams are recharged by groundwater seepage. The latter is important in increasing the low water discharge during dry periods. Periods of high precipitation and the sloping land surface often make it necessary to build retarding basins as flood control reservoirs. The subject of this paper is to investigate the consequences when the high-water of the streams is used for artificial groundwater recharge thus eliminating the need for expensive retarding basins. It is proposed that existing or newly constructed gravel-pits could be built in areas where the water table is located deep beneath the surface. REGION OF INVESTIGATION For this investigation the Eschach, Wurzacher Ach and Aitrach streams near Leutkirch, West Germany, were chosen as a case study (Figs 1, 2). These streams flow into the River Iller which empties into the Danube. In particular, the Eschach stream is known to be very susceptible to severe high-water conditions after intense rainfalls 33 34 B.Herrling or during spring thaws, which endanger the town of Leutkirch. The average streamflow in the Eschach stream is 2 m3s-1 (1957-1978), reaching maximum values of 52 m3s-1 upstream of Urlau (Fig.2) during peak periods. The drainage area amounts to nearly 120 km at Leutkirch. Retarding areas which have been used in the past will soon be no longer available and the construction of a new retarding basin in this geographical area poses considerable complications. It is, however, well known that the aquifer in the valley of the Eschach FIC.1 General plan of the area of interest. stream is very conductive (K < 0.02 m s"" in connection with a saturated thickness of sometimes more than 20 m). Furthermore the distance between the water table and the surface can locally be more than 20 m. Consideration has therefore been given to artificial groundwater recharge in order to raise the groundwater levels (Werner et al., 1974; Wrobel & Werner, 1979) and thus alleviate the demand for retarding basins. The fundamental idea consists in using newly constructed or existing gravel-pits as recharge basins and filling these with the high-waters of the Eschach stream. As these pits have a very permeable bottom as opposed to the relatively dense stream bed, the water would quickly seep into the ground thus removing the danger of downstream flooding. In non-peak periods the recharged water could be stored, and could possible contribute to higher discharges in the Aitrach stream and the River II1er during dry periods. This would in fact very likely be the case since the water table comes up to the surface near the Wurzacher Ach stream. The future amount of drinking water available to nearby regions with a high density of population could also be significantly increased by drawing as much water as possible from the Eschach stream, leaving behind only a minimum streamflow. This high quality water could be tapped from a. suitable location downstream of the recharge basin. A preliminary investigation was carried out by the State Artificial groundwater recharge 35 FIG.2 Site plan of the area of investigation. Government of Baden-Wiirttemberg to create a network of hydrological and groundwater data points. Also, numerous boreholes were drilled in order to yield information about groundwater levels and the geology of the area. In a second step, a small test pit 7 m deep was constructed which spreads over an area of 100 m2 at the bottom and of 1400 m at the surface. In 1982 artificial groundwater recharge is planned using the water of the Eschach stream. The simultaneous measurement of more than 40 parameters in the neighbourhood of the pit will yield information about the water flux and the seepage process. The latter is of particular interest because of the stratified sediments with changing hydraulic conductivities which underlie the pit. The author's intentions are to simulate these seepage processes with a numerical model. Further research is to be directed towards the downstream consequences of the artificial groundwater recharge. Here the possibility of waterlogging will be examined and if necessary further preventive measures. Additionally, the effect of the runoff in the Aitrach stream, which is considerably influenced by effluent groundwater, is to be studied. For these reasons a numerical model has been set up to simulate the groundwater flow. In a first step the model was calibrated in its steady state form (Brandstetter, 1979) using an average hydrological situation and corresponding measured groundwater levels. In a further step the model has been run for time-dependent conditions as described in the next section. 36 B.Herrling COMPUTATION OF THE TIME-DEPENDENT, HORIZONTAL GROUNDWATER FLOW Basic theory The basic equations describing the time-dependent, two-dimensional horizontal groundwater flow in an unconfined aquifer are the continuity equation Sy h,t + qiri = q* (1) and Darcy's equation of motion 9i = ~Tij h'j (2) summing over i,j = 1, 2. These equations result from a vertical integration using the Dupuit assumption. The unknown parameters are the groundwater level h (height of water table) and the horizontal flux per unit length q. defined in a Cartesian coordinate system x.. The symbols (),.; and (),t denote partial differentiations with respect to the coordinates Xj_ and the time t. An inflow in the area of computation is specified by q*. In addition there is the specific yield S,, for the free surface flow and the transmissivity tensor T. •. y -* 2.J When equation (2) is set into the continuity equation (1) it results in Sy h't~ (Tij h'-j>'i = ^ (3) computed in a second step when h is known. In this case the method of weighted residuals is the basis of the application of the finite element analysis. In time, the equations are solved in a stepwise manner as usual, starting with a given initial condition. The domain of computation is subdivided into finite elements for the integration of the weighted basic equation (3). SJ, oh Sw h,. dt dA - Ij,f. ôh (T.. h;i),. dt dA = IS ôh q dt dA + /g/t ôh q dt dS + ft 6hK QK dt (4) There are chosen elements in space (triangles) and time with linear shape functions respectively. The weighting functions ôh are also functions in space and time. In space they are identical with the shape functions (Galerkin method), and in time, constant weighting functions are chosen which give the same factors as the Crank-Nicolson time integration. The previous inflow q* is replaced by an areal distributed groundwater increment q, e.g. as a consequence of the precipitation, by a line source of mass inflow q defined on element boundaries S and by a point source of mass inflow QK at node K of the element system. Equation (4) is transformed by partial integration. Using Gauss' integral theorem and employing again equation (2) this results in: Artificial groundwater recharge 37 fj, ôh S„ h,. dt dA + f.f- ôh, . T. h, . dt dA + AC y !• A "C 1 1J J /rX- ôh q, n. dt dS = /,/, ôh q dt dA + /„/. ôh q dt dS + bull A L. ù "C ;t ShK QK dt (5) Boundary conditions are the hydrographs of groundwater level or of the flux across the boundaries. h - h = 0 (6) qini + i = ° <7> h signifies a prescribed water level and q a flux normal to the outer boundaries. On closed boundaries q is equal to zero. While equation (6) is considered in the final set of equations, the condition (7) is inserted directly into equation (5) and can be combined with the second term on the right side of the sign of equality. In horizontal models it is very important to consider the water exchange with the streams. Therefore a line source term (leakage term) is formulated q' = X • B • (hs - h) (8) with the leakage parameter X, which signifies the quotient from the permeability and the thickness of the bottom layer of a stream, together with the width B and the water level h^ of a stream. The leakage term (8) is treated as the other line source term.