Lecture Packet #6: Groundwater-Surface Water Interactions

1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet #6: Groundwater-Surface Water Interactions Streams • Streamflow is made up of two components: o Surface water component ƒ Surface Runoff ƒ Direct Rainfall o Groundwater component (baseflow) ƒ Seepage through the streambed or banks • Definitions o Losing or influent stream Æ Stream feeds aquifer o Gaining or effluent stream Æ Aquifer discharges to stream Gaining Stream Losing Stream Water Table 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 1 of 10 Stream-Aquifer Interactions t0 tP Stream Discharge 0 Recharge Flood hydrograph with bank storage 0 t0 Discharge Flood hydrograph with no bank storage Bank Storage Volume Stream Discharge Base Flow – Contribution to streamflow from groundwater • Upper reaches provide subsurface contribution to streamflow (flood wave). • Lower reaches provide bank storage which can moderate a flood wave. tP Water leaving bank storage 0 Water entering bank storage Groundwater inflow and bank storage Groundwater inflow alone 0 t0 tP Springs • A spring (or seep) is an area of natural discharge. • Springs occur where the water table is very near or meets land surface. o Where the water table does not actually reach land surface, capillary forces may still bring water to the surface. • Discharge may be permanent or ephemeral • The amount of discharge is related to height of the water table, which is affected by o Seasonal changes in recharge o Single storm events • • • • • • Depression spring Contact spring Fault spring Sinkhole spring Joint spring Fracture spring Types of Springs Land surface dips to intersect the water table Water flows to the surface where a low permeability bed impedes flow Similar to a depression spring, but flow is limited to fracture zones, joints, or dissolution channels 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 2 of 10 Regional Groundwater Flow Depression Spring Contact Spring Fault Spring Spring Spring Spring Sinkhole Spring Joint Springs Spring Fracture Spring Spring Spring Spring 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 3 of 10 Where does water come from when pumping? • • • • • • Initial rate of recharge balances initial rate of discharge. Water pumped comes from storage and recharge within cone of depression. Water pumping creates cone of depression reaches shoreline. Ultimate magnitude of pumpage (before well dries up – at the well) is dependent on hydraulic conductivity, thickness, available drawdown. Ultimate production of water depends upon how much rate of recharge can be changed and/or how much water can be captures. Steady-state production is not dependent on Sy. Although rate of recharge = discharge is interesting, it is almost irrelevant in determining the sustained yield of the aquifer. (Here, think of case where rainfall is small or does not exist – water source is ultimately the lake.) 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 4 of 10 General Conclusions: Essential factors that determine response of the aquifer to well development • • • Distance to, and character of, recharge (precip vs. pond) Distance to, and character of, natural discharge Character of cone of depression (function T and S) Prior to development aquifer is in equilibrium. “All water discharged by wells is balanced by a loss of water from somewhere.” -Theis (1940) When pumping occurs, water comes from storage until a new equilibrium is reached. Accomplished by: • • • Increase in recharge Æ capture of a water source Decrease in discharge Æ reduction of gradient Æ outflow Both Some water must always be mined (taken from storage) to create groundwater development. Mathematically the pumping water balance is: Q = (R + ∆R) – (D + ∆D) – S(∆h/∆t) If over the years R = D, and a new equilibrium (new steady state) is reached (∆h/∆t = 0) Q = ∆R - ∆D 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 5 of 10 Valley of Large Perennial Stream in Humid Region Large Perennial Stream Water Table Alluvium (sand and gravel) 150 ft Bedrock 5 miles Setting – East Coast • Thick, permeable alluvial valley cut into shale • Large perennial stream • Shallow water table with many phreatophytes (trees that can stick roots below water table and saturate their roots) • Moderately heavy precipitation Sources of Water • Withdrawal from storage (cone of depression) • Salvaged rejected recharge (prevention of runoff to stream by making more room for recharge from precipitation – water goes to groundwater rather than stream – only some of stream water recharges. • Salvaged natural discharge (natural discharge w/o pumping) o Lowering water table beneath phreatophytes o Decreasing gradient toward stream decreasing base flow (for low development rates) – river is a GW sing – a gaining stream under natural conditions • Over long term at steady state Q = ∆R + ∆D Small developments ∆R source Å Room for precipitation Large developments, stream capture, ∆D, source. 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 6 of 10 Valley of Ephemeral Stream in Semiarid Region Ephemeral or Intermittent Stream Water Table Alluvium (sand and gravel) 100 ft Bedrock 1 mile Setting – West Coast (not northwest) • Moderately thick, permeable alluvial valley cut into shale • Large ephemeral stream • Water table beneath stream channel, below vegetation • Precipitation like in Palo Alto, about 15 in/yr • Stream dry most of year, floods in heavy rains Sources of Water • Withdrawal from storage (cone of depression) • No salvaged rejected recharge (enough room for all recharge from low precipitation) • Little salvaged natural discharge (no phreatophytes) • Recharge directly from stream (water table low enough so that there is room for flood waters – evaporation-free-control reservoir) – can guarantee this with pumping • ∆D = 0 o Capture floodwaters and get + S ∆h/∆t When ∆R = 0 loss from storage is only source Q = -S ∆h/∆t (water can be pumped seasonally for irrigation and later replenished) 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 7 of 10 High Plains of Texas and New Mexico Water Table Springs and seeps Ogaliala Formation 500 ft Silt, sand & gravel Triassic and older rocks 150 miles Setting • Remnant high plain sloping, cut off from external sources of water by escarpments both upgradient and downgradient • Thick (300 ft to 600 ft) permeable rocks on impermeable rocks • Recharge from precipitation is 1/20 to 1/2 in/yr • Discharge from springs about the same • Water table (>50 ft) Sources of Water • Withdrawal from storage (cone of depression) • No salvaged rejected recharge (ample space – 50 ft. unsaturated) • Little salvaged natural discharge (gradient unchanged, but even if not, aquifer flow would only account for 1 to 2% of the withdrawal rate) • Q = -S ∆h/∆t water from storage – mining only Sy = 0.15 • Big difference with Entrada Sandstone S = 5 x 10-5 per square foot for each foot of head decline 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 8 of 10 Productive Artesian Aquifer System Ephemeral Stream Potentiometric surface Siltstone and mudstone 1000 ft Entrada Sandstone 5 miles Setting • Grand Junction Artesian Basin, Colorado • Typical low conductivity artesian aquifer • Fine-grained sandstone, party cemented with calc. carb. • 150 ft. thick Æ T = 20 ft2/d, S = 5 x 10-5 • Recharge from precipitation (7-8 in/yr) where outcrops are in contact with alluvium • Discharge small and from upward leakage through relatively impermeable siltstone 500 to 1000 ft thick • Artesian conditions, as much as 160 ft above land surface Sources of Water • Withdrawal from confined storage (large overlapping cones of depression) • No salvaged rejected recharge – already room for recharge water; no extra would enter aquifer if water table in recharge area were lowered. (limiting unit is artesian aquifer) • Little salvaged natural discharge (limited upward leakage) • Acts like confined “bathtub” with little ∆D due to pumping. • Q = -S ∆h/∆t water from storage – mining only – mining artesian storage and not dewatering storage 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 9 of 10 Closed Desert Basin Ephemeral Stream Water Table Bedrock A B 1000 ft Alluvium (silt, sand, and gravel) 20 miles Setting • Thick coalescing alluvial fans, gradational from mountains • Basin receives precipitation of 3-5 in/yr, mountains 20-30 in/yr • Very shallow water table near playa, deep near mountains • Streams are ephemeral • Phreatophytes near playa Sources of Water • Withdrawal from storage (create cone of depression) • Salvaged rejected recharge (center none – precip in valley evaporates or transpires) (border some recharge from small ephemeral streams near surrounding mountains) • Salvaged natural discharge o Lowering water table near playa may reduce ET (roots) o Near borders of basin discharge toward playa can be reduced (stop flow to center where ET occurs) • Operation – increase rejected recharge and prevent existing discharge: Q = ∆R + ∆D – (+/- S ∆h/∆t) • Retention dams to capture flood waters for recharge 1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet 6 Page 10 of 10