8. Regional groundwater system

Transcription

1 8. Regional groundwater system 8-1 We have learned basic principles governing the flow and storage of groundwater. We will now use these principles to understand groundwater in the regional context. Flow equation in vertical cross section. Vertical cross sections are commonly used to understand the regional groundwater flow. The steady-state flow equation in a vertical cross section is: K x x h x + K z z h z = 0 [8-1] This equation is useful when the flow pattern is symmetric. Symmetry of flow system Flow analysis is greatly simplified when the flow symmetry is present. This diagram shows the groundwater flowing into a reasonably straight stream section. Will groundwater flow in section be similar to B? B ny symmetries in this system?

2 Streams and drainage divide are common example of the plane of symmetry. If the climate and geology is homogeneous, groundwater does not flow across the symmetry plane Flownet analysis in vertical cross sections We can graphically solve Eq. [8-1] using the techniques of flownet analysis (see p.6-18). Suppose a cross section of homogeneous, isotropic material. Can we construct a flow net for this case? 10 elevation (m) h = 10 m h = 7 m m Useful tip: Draw the flow lines and equipotentials so that they make squares (see next page).

3 There are five and a third flow tubes. Suppose K = 10-4 m/s. What is the total Q for one meter of the cross section? Flow nets with water table Recall that ψ = 0 and h = z at the water table. ny point on the water table can be used as the control point for equipotential (for example, h = 9 m). Can we construct a flow net? Pay attention to the areas of groundwater recharge and discharge. 10 h = 9 elevation (m) m

4 The flow net we just made is called Hubbert section due to one of the giants in hydrogeology. 8-4 (Hubbert, M.K., Theory of groundwater motion. Journal of Geology, 48: ). B B impermeable bed rock The Hubbert section represents a model of regional groundwater flow system. Both and Β are the planes of symmetry and act as no flow boundaries. Note that vertical direction of flow is downward under recharge areas (hills) and upward under discharge areas (valleys and depressions). In other words, h increases with depth under discharge area.

5 Piezometers and equipotentials In the field equipotentials are inferred based on a few piezometer data. Hydrogeologists need to develop a good sense of flow system because field data are usually very limited. z = 970 m h = 970 m, equipotential B 8-5 Note that the piezometer at B is under the flowing condition. This is commonly observed near rivers and lakes. Undulating water table The undulating water table results in complex flow patterns having multiple recharge and discharge areas. short-range flow system (shaded region) is superimposed on top of a longer-range flow system. 10 elevation (m) m

6 Regional and local flow systems 8-6 The undulating water table occurs under the undulating terrain. In this case, the local flow system is superimposed on top of the regional flow system This idea was first proposed in 1963 by J. Toth based on his observation of groundwater near Red Deer and quickly became the world-wide standard. (J. Geophys. Res., 68: ) m elevation (m) m km

7 The flow net in the last page is drawn for the homogeneous and isotropic case, but flow lines are not normal to equipotentials. Why? vertical exaggeration 8-7 Effects of anisotropy In anisotropic materials flow lines are generally not normal to equipotentials. When the material has a high ratio of K x to K z, the flow paths tend to be shallower. K x /K z = 1 K x /K z = 5

8 Effects of heterogeneity 8-8 When relatively low-k material is underlain by a high-k aquifer, water flows vertically through low-k material and horizontally through the aquifer. high K (100 ) layer This is situation is commonly encountered in the prairies, where the clay-rich glacial till is underlain by inter-till aquifers, and is called prairie profile. saline soil aquifer as an under drain The Prairie Profile. (Meyboom. 1966, Geol. Survey of Canada Bulletin 139) When an aquifer pinches out, discharge becomes intensified above the edge of the aquifer and causes soil salinization. high K (100 ) lens

9 Interaction of groundwater with surface water 8-9 Groundwater outcrops at lakes, streams, and springs. The direction of flow is most conveniently indicated by the water table map. gaining stream loosing stream water table contours What is the elevation of stream water surface at? Groundwater has several important ecological functions: (1) It maintains the baseflow during dry periods. (2) It regulates the temperature of water around springs. (3) It brings nutrients (carbon, nitrogen, etc.) into the stream. (4) It supports stream-side (riparian) vegetation; In arid and semi-arid regions intermittent streams and ponds contribute significantly to recharging shallow groundwater. Suggested reading 1. Ground water and surface water: single resource ( 2. Effects of ground water exchange on the hydrology and ecology of surface water (posted on the course home page)