Stream-Aquifer Interactions Base Flow – Contribution to streamflow from groundwater

  1.72, Groundwat er Hydrology Prof. Charles Harvey

  

Le ct u r e Pa ck e t # 6 : Gr ou n dw a t e r - Su r fa ce W a t e r I n t e r a ct ion s

St r e a m s

• Stream flow is m ade up of two components: _o

  Surface water com ponent ƒ

  Surface Runoff ƒ _o Direct Rainfall

  Groundwater com ponent (baseflow) ƒ

  Seepage through the stream bed or banks

• Definitions _o

Losing or influent st ream Æ o Stream feeds aquifer

  Gaining or effluent st ream Æ Aquifer discharges t o st ream Gaining St ream Losing St ream

  Wat er Table

  St r e a m - Aqu ife r I n t e r a ct ion s Ba se Flow – Cont ribut ion t o st ream flow from groundw at er • Upper reaches provide subsurface contribution to stream flow (flood wave).

• Lower reaches provide bank storage which can m oderate a flood wave.

  e Flood hydr ogr aph w it h no rg e bank st orage a rg h a c Wat er leaving bank st or age h is

  Flood hydr ogr aph w it h c

  D is bank st orage D m e a e rg a tr h Wat er ent er ing bank st or age S c

  t t P

  e R e e rg

  Groundw at er inflow and g a bank st orage h e ra c m to is

  Groundw at er inflow alone lu S

  D o k

  V m n a a e B tr S

  t t t P t P

  Spr in gs • A spring (or seep) is an area of natural discharge.

• Springs occur where the water table is very near or m eets land surface. _o

  Where the water table does not actually reach land surface, capillary forces m ay st ill bring w at er t o t he surface.

• Discharge m ay be perm anent or ephem eral
• The am ount of discharge is related to height of the water table, which is affect ed by _{o o}

  Seasonal changes in recharge Single storm events

  Types of Springs Land surface dips t o int ersect t he w at er t able

• Depression spring

  Wat er flow s t o t he surface w here a low perm eabilit y bed

• Contact spring im pedes flow
• Fault spring

  Sim ilar t o a depression spring, but flow is lim it ed t o

• Sinkhole spring fract ure zones, j oint s, or dissolut ion channels
• Joint spring
• Fracture spring

  Re giona l Gr ou ndw a t e r Flow Depr ession Spr ing Cont act Spr ing Fault Spr ing Spring

  Spring Spring Sinkhole Spr ing Joint Spr ings Fract ure Spr ing

  

Spring

Spring Spring Spring

W h e r e doe s w a t e r com e fr om w h e n pu m pin g? • I nitial rate of recharge balances initial rate of discharge

• Water pumped com es from storage and recharge within cone of depression.
• Water pumping creates cone of depression reaches shoreline.
• Ultim ate m agnitude of pum page (before well dries up – at the well) is dependent on hydraulic conduct ivit y, t hickness, available draw dow n.
• Ultim ate production of water depends upon how m uch rate of recharge can be changed and/ or how m uch w at er can be capt ures. St eady- st at e product ion is not dependent on S y .
• Although rate of recharge = discharge is interesting, it is alm ost irrelevant in det erm ining t he sust ained yield of t he aquifer. ( Here, t hink of case where rainfall is sm all or does not exist – w at er source is ult im at ely t he lake.)

  Ge n e r a l Con clu sion s: Esse n t ia l fa ct or s t h a t de t e r m in e r e spon se of t h e a qu ife r t o w e ll de ve lopm e n t

• 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 t o developm ent aquifer is in equilibrium . “ All w at er discharged by w ells is balanced by a loss of w at er from som ew here.”
• Theis ( 1940) When pum ping occurs, w at er com es from st orage unt il a new equilibr ium is reached. Accom plished by:
• I ncrease in recharge Æ capture of a water source
• Decrease in discharge Æ reduction of gradient Æ outflow
• Both Som e wat er m ust always be m in e d ( t aken from st orage) t o creat e groundw at er developm ent . Mat hem at ically t he pum ping w at er balance is: Q = ( R + ∆R) – (D + ∆D) – S(∆h/ ∆t) I f over t he years R = D, and a new equilibr ium ( new st eady st at e) is reached

  ∆h/ ∆t = 0) (

  Q = ∆R - ∆D

  V a lle y of La r ge Pe r e n n ia l St r e a m in H u m id Re gion Lar ge Per ennial St ream

  Wat er Table Alluvium 150 ft ( sand and grav el)

  Bedrock 5 m iles Se t t in g – Ea st Coa st

• Thick, perm eable alluvial valley cut into shale
• Large perennial stream
• Shallow water table with m any phreatophytes (trees that can stick roots below w at er t able and sat urat e t heir root s)
• Moderately heavy precipitation

  Sou r ce s of W a t e r

• Withdrawal from storage (cone of depression)
• Salvaged rejected recharge (prevention of runoff to stream by m aking m ore room for recharge from precipit at ion – wat er goes t o groundwat er rat her t han st ream – only som e of st ream wat er recharges.
• Salvaged natural discharge (natural discharge w/ o pum ping) _{o o}

  Lowering water table beneath phreatophytes Decreasing gradient toward stream decreasing base flow (for low developm ent rat es) – river is a GW sing – a gaining st ream under nat ural condit ions

• Over long term at steady state

  Q = ∆R + ∆D Sm all developm ent s

  ∆R source Å Room for precipitation Large developm ent s, st ream capt ure, ∆D, source.

  V a lle y of Eph e m e r a l St r e a m in Se m ia r id Re gion Ephem eral or I nt er m it t ent

  St ream Wat er Table Alluvium 100 ft

  ( sand and grav el)

Bedrock

1 m ile

  Se t t in g – W e st Coa st ( n ot n or t h w e st )

• Moderately thick, perm eable alluvial valley cut into shale
• Large ephem eral stream
• Water table beneath stream channel, below vegetation
• Precipitation like in Palo Alto, about 15 in/ yr
• Stream dry m ost of year, floods in heavy rains

  Sou r ce s of W a t e r

• Withdrawal from storage (cone of depression)
• No salvaged rejected recharge (enough room for all recharge from low precipit at ion)
• Little salvaged natural discharge (no phreatophytes)
• Recharge directly from stream (water table low enough so that there is room for flood w at ers – evaporat ion- free- cont rol reservoir) – can guarant ee t his w it h pum ping
• ∆D = 0 _o

  Capture floodwaters and get + S ∆h/ ∆t When ∆R = 0 loss from storage is only source

  ∆h/ ∆t (water can be pum ped seasonally for irrigat ion and later Q = - S replenished)

  H igh Pla in s of Te x a s a n d N e w M e x ico Wat er Table Springs and seeps

  Ogaliala For m at ion 500 ft Silt , sand & gravel

  

T riassic and older rocks

150 m iles

Se t t in g

• Rem nant high plain sloping, cut off from external sources of water by escarpm ent s bot h upgradient and dow ngradient
• Thick (300 ft to 600 ft) perm eable rocks on im perm eable rocks
• Recharge from precipitation is 1/ 20 to 1/ 2 in/ yr
• Discharge from springs about the sam e
• Water table (> 50 ft)

  Sou r ce s of W a t e r

• 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 w ould only account for 1 t o 2% of t he w it hdraw al rat e) _y

  = 0.15

• Q = -S ∆h/ ∆t water from storage – m ining only S _{- 5} per square foot for each
• Big difference with Entrada Sandstone S = 5 x 10 foot of head decline

  Pr odu ct ive Ar t e sia n Aqu ife r Syst e m Ephem eral St r eam Pot ent iom et r ic surface

  Silt st one and m udst one 1000 ft Ent rada Sandst one 5 m iles

  Se t t in g

• Grand Junction Artesian Basin, Colorado • Typical low conductivity artesian aquifer • Fine-grained sandstone, party cemented with calc. carb. _{2 - 5}

  / d, S = 5 x 10

• 150 ft. thick Æ T = 20 ft
• Recharge from precipitation (7-8 in/ yr) where outcrops are in contact with alluvium
• Discharge sm all and from upward leakage through relatively im perm eable silt st one 500 t o 1000 ft t hick
• Artesian conditions, as m uch as 160 ft above land surface

  Sou r ce s of W a t e r

• Withdrawal from confined storage (large overlapping cones of depression)
• No salvaged rejected recharge – already room for recharge water; no extra w ould ent er aquifer if wat er t able in recharge area w ere low ered. ( lim it ing unit is art esian aquifer)
• Little salvaged natural discharge (lim ited upward leakage) • Acts like confined “bathtub” with little ∆D due to pum ping.
• Q = -S ∆h/ ∆t water from storage – m ining only – m ining artesian storage and not dew at ering st orage

  Close d D e se r t Ba sin Ephem eral St r eam Bedrock

  Wat er Table A B 1000 ft

  Alluvium ( silt , sand, and gravel) 20 m iles

  Se t t in g

• Thick coalescing alluvial fans, gradational from m ountains
• Basin receives precipitation of 3-5 in/ yr, m ountains 20-30 in/ yr
• Very shallow water table near playa, deep near m ountains
• Stream s are ephem eral
• Phreatophytes near playa

  Sou r ce s of W a t e r

• Withdrawal from storage (create cone of depression)
• Salvaged rejected recharge (center none – precip in valley evaporates or t ranspires) ( border som e recharge from sm all ephem eral st ream s near surrounding m ount ains)
• Salvaged natural discharge _{o o}

  Lowering water table near playa m ay reduce ET (roots) Near borders of basin discharge toward playa can be reduced (stop flow t o cent er w here ET occurs)

• Operation – increase rejected recharge and prevent existing discharge:

  ∆R + ∆D – (+ / - S ∆h/ ∆t) Q =

• Retention dam s to capture flood waters for recharge