Groundwater and surface water: a single resource. Santa Cruz River, Tucson

Transcription

1 Groundwater and surface water: a single resource Santa Cruz River, Tucson

2 Groundwater basics Groundwater is water found within the pore spaces of geologic material beneath the surface of the Earth. It exists in saturated layers of sands and gravels, in certain types of clay material, and in cracks within crystalline rock. -Cech, 2002 Are all pore spaces created equal? 2

3 Porosity in different sediments. A. A porosity of 30 percent in a reasonably well-sorted sediment. B. A porosity of 15 percent in a poorly sorted sediment in which fine grains fill spaces between larger grains. C. Reduction in porosity in an otherwise very porous sediment due to cement that binds grains together. 3

4 Pore Spaces Sand Grav el A B C Clay D E F Typical grain packing showing: A and B) well sorted, well rounded grains; C) poorly sorted, well rounded grains; D) moderately well sorted, angular grains; E) platy uncompressed grains; and F) platy compressed grains. Ferre,

5 Typically, the water table separates the zone of aeration (vadose zone) from the saturated zone and fluctuates in level with seasonal changes in precipitation. Corresponding fluctuations are seen in the water level in wells that penetrate the water table. Lakes, marshes, and streams occur where the water table intersects the land surface. In shape, the water table is a subdued imitation of the overlying land surface. 5

6 Simplified View Components of subsurface water: Which way will water flow? 6

7 Groundwater Recharge Precipitation and surface water infiltrate below ground until intercepted by plant roots or slowed down by a less permeable material such as clay, shale or bedrock. This naturally occurring process of downward water migration that reaches the groundwater table is called groundwater recharge. Cech,

8 Aquifers An aquifer is a water-bearing geologic formation that can store and yield usable amounts of water. Latin orgin: aqua = water ferre = to bear or carry -Cech,

9 Aquifer terms Unconfined aquifer = Aquifer with no confining bed of material between the saturated zone and the land surface. Water table = the surface of the unconfined aquifer Confined aquifer = Aquifer that is overlain by a confining bed of geologic material Cech,

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11 Aquifer terms Permeability = the measure of the ability of a material to transmit fluids through it. Wikipedia, 2006 Aquitard = Geologic material with a relatively low permeability that yields low amounts of groundwater. - Freeze & Cherry, 1979 Examples: clay and bedrock Aquiclude = Impermeable geologic material that prevents groundwater movement. - Cech, 2002 Examples: highly compacted clays & bedrock 11

12 Basin and Range Aquitard Aquifer Aquiclude B. Scarborough, Desert Museum, 2002 capp.water.usgs.gov/gwa/ch_c/c-basin.html 12

13 Confined Aquifer Aquitard 13

14 Confined aquifer versus unconfined aquifer (plus respective water elevations in wells) Water elevation in unconfined aquifer wells = water table elevation Unconfined Aquifer Water table 14

15 The High Plains aquifer is an example of an unconfined aquifer. A. This section across southeastern Wyoming and central Nebraska shows the eastward slope of the water table and the relation of the aquifer to underlying rock units. 15

16 Aquifer QUIZ! What is this level called? a) Unsaturated What zone is zone this? b) Saturated What zoneis this? c) This is called? d) This is called? a.k.a. Aquitard what? Name of well is? 16

17 Aquifer QUIZ! What is this Water Table level called? a) Unsaturated What zone is zone this? b) Saturated What zoneis this? c) Unconfined This is called? d) This Confined is called? a.k.a. Aquitard what? Name Artesian of wellis? 17

18 Formation of cone of depression Note: only occurs in unconfined aquifer 18

19 How does groundwater flow? 19

20 Flow: Elevation Pressure, head No Flow Flow 20

21 How does groundwater flow? Above ground: Water flows downhill! Below ground: Water flows down gradient Groundwater naturally moves to lower elevations due to the force of gravity. However, the direction and rate of movement are determined by lithology, stratigraphy, and structure of geologic deposits. Cech,

22 Darcy Flow and Conductivity Q h --- = -k * A l Freeze & Cherry, 1979 Q = flow K = hydraulic conductivity h = hydraulic head = elevation + pressure l = length A = cross-sectional area Factors that affect flow or conductivity Sediments and their packing (permeability) Fluid properties such as viscosity and density Orders of magnitude of K (m/s) Clay ~ 10-12, Slit ~ 10-8, Sand ~ 10-4, Gravel ~

23 Permeability Relative Permeability Pervious Semi-Pervious Impervious Unconsolidated Sand & Gravel Well Sorted Gravel Well Sorted Sand or Sand & Gravel Very Fine Sand, Silt, Loess, Loam Unconsolidated Clay & Organic Peat Layered Clay Unweathered Clay Consolidated Rocks Highly Fractured Rocks Oil Reservoir Rocks Fresh Sandstone Fresh Limestone, Dolomite Fresh Granite -Wikipedia, 2006 and Bear, Jacob, Dynamics of Fluids in Porous Media, Dover. 23

24 Groundwater and surface water: Travel times 24

25 Geology break: stream cross-section More simply: bench terrace stream 25

26 Recharge in mountain areas Water from mountaintop precipitation can follow several pathways: A. Infiltration (slowest) B. Transport in shallow water zone C. Overland flow (fastest) 26

27 Water Tables & Flow Contouring groundwater from well data how do hydrologists know which way water is flowing? Location of wells in map view (above) Contours of equal hydraulic head may be drawn from water levels in wells Flow lines are perpendicular to countour lines 27

28 Interactions: Gaining Streams Gaining streams streams that receive water from the groundwater system Contour plot of a gaining stream This can be determined from water-table contour maps because the contour lines point in the upstream direction where they cross the stream 28

29 Interactions: Losing streams Losing streams streams that lose water to the groundwater system Contour plot of a losing stream can be determined from watertable contour maps because the contour lines point in the downstream direction where they cross the stream 29

30 Groundwater-surface water interactions Influence of increased surface water flow on groundwater During high flow, stream water moves into the unsaturated portion of the stream banks as bank storage 30

31 Groundwater-surface water interactions Impact of increased surface water flow A. Initially gaining stream B. Flooding causes stream to rise, bank storage occurs C. Additional bank storage water recharges groundwater (losing conditions) increasing height of the water table 31

32 Human impacts on groundwatersurface water resources 32

33 Development of sinkholes, subsidence Sinkhole in Florida caused by overpumping of groundwater for irrigation Subsidence fissure in Arizona caused by groundwater pumping 33

34 Subsidence WRRC, University of Arizona,

35 Impacts: Groundwater-stream interactions Drawdown of aquifer causes Santa Cruz River to dry out and destroys riparian vegetation along stream banks 35

36 Impact: Groundwater / surface water interactions Gaining stream: g.w. discharges into stream naturally Installation of a well and pumping may intercept water discharging to stream Increased pumping rates may draw water from the stream itself, creating losing stream conditions 36

37 GW-SW interaction - San Pedro Natural cycle: groundwater recharges river Today: increased groundwater withdrawls influence stream 37

38 Impact: pollution GW flow dir 38

39 Impact: saltwater intrusion Coastal areas: freshwater flows over saltwater because it is less dense Large draw downs of aquifers can cause underlying saltwater to migrate upwards into wells 39