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
• St ream flow is m ade up of t w o com ponent s: o Surface w at er com ponent
Surface Runoff Direct Rainfall
o Groundw at er com ponent ( baseflow )
Seepage t hrough t he st ream bed or banks
• Definit ions
o Losing or influent st ream ÆSt ream feeds aquifer
o Gaining or effluent st ream Æ Aquifer discharges t o st ream
Wat er
Losing St ream Gaining St ream
B
• Upper reaches provide subsurface cont ribut ion t o st ream flow ( flood w ave) .
• Lower reaches provide bank st orage which can m oderat e a flood wave.
Flood hydr ogr aph w it h no
Groundw at er inflow alone
Spr in gs
• A spring ( or seep) is an area of nat ural discharge.
• Springs occur where t he w at er t able is very near or m eet s land surface. o Where t he w at er t able does not act ually reach land surface, capillary
forces m ay st ill bring w at er t o t he surface.
• Joint spring fract ure zones, j oint s, or dissolut ion channels
• Fract ure spring
1.72, Groundw at er Hydrology Lect ure Packet 6
Re giona l Gr ou ndw a t e r Flow
Spring Spring
Spring
Spring
Spring Spring Spring
Fault Spr ing Cont act Spr ing
Depr ession Spr ing
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 nit ial rat e of recharge balances init ial rat e of discharge.
• Wat er pum ped com es from st orage and recharge w it hin cone of depression.
• Wat er pum ping creat es cone of depression reaches shoreline.
• Ult im at e m agnit ude of pum page ( before w ell dries up – at t he w ell) is dependent on hydraulic conduct ivit y, t hickness, available draw dow n.
• Ult im at e product ion of w at er depends upon how m uch rat e 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 Sy.
• Alt hough rat e of recharge = discharge is int erest ing, 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.)
1.72, Groundw at er Hydrology Lect ure Packet 6
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
• Dist ance t o, and charact er of, recharge ( precip vs. pond)
• Dist ance t o, and charact er of, nat ural discharge
• Charact er of cone of depression ( funct ion 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 Æcapt ure of a w at er source
• Decrease in discharge Æ reduct ion of gradient Æ out flow
• Bot h
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)
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
• Thick, perm eable alluvial valley cut int o shale
• Large perennial st ream
• Shallow w at er t able w it h m any phreat ophyt es ( t rees t hat can st ick root s below w at er t able and sat urat e t heir root s)
• Moderat ely heavy precipit at ion
Sou r ce s of W a t e r
• Wit hdraw al from st orage ( cone of depression)
• Salvaged rej ect ed recharge ( prevent ion of runoff t o st ream 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 nat ural discharge ( nat ural discharge w / o pum ping) o Low ering wat er t able beneat h phreat ophyt es
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
• Moderat ely t hick, perm eable alluvial valley cut int o shale
• Large ephem eral st ream
• No salvaged rej ect ed recharge ( enough room for all rechar ge from low precipit at ion)
• Lit t le salvaged nat ural discharge ( no phreat ophyt es)
H igh Pla in s of Te x a s a n d N e w M e x ico escarpm ent s bot h upgradient and dow ngradient
• Thick ( 300 ft t o 600 ft ) perm eable rocks on im perm eable rocks
Pr odu ct ive Ar t e sia n Aqu ife r Syst e m
• Grand Junct ion Art esian Basin, Colorado
• Typical low conduct ivit y art esian aquifer
• Fine- grained sandst one, part y cem ent ed w it h calc. carb.
• 150 ft . t hick Æ T = 20 ft2
/ d, S = 5 x 10- 5
• Recharge from precipit at ion ( 7- 8 in/ yr) w here out crops are in cont act w it h alluvium
• Discharge sm all and from upward leakage t hrough relat ively im perm eable silt st one 500 t o 1000 ft t hick
• Art esian condit ions, as m uch as 160 ft above land surface
Sou r ce s of W a t e r
• Wit hdrawal from confined st orage ( large overlapping cones of depression)
Close d D e se r t Ba sin
• Thick coalescing alluvial fans, gradat ional from m ount ains
• Basin receiv es precipit at ion of 3- 5 in/ yr, m ount ains 20- 30 in/ yr
• Very shallow w at er t able near playa, deep near m ount ains
• St ream s are ephem eral
• Phreat ophyt es near playa
Sou r ce s of W a t e r
• Wit hdraw al from st orage ( creat e cone of depression)
• Salvaged rej ect ed recharge ( cent er none – precip in valley evaporat es or t ranspires) ( border som e recharge from sm all ephem eral st ream s near surrounding m ount ains)
• Salvaged nat ural discharge
o Lowering wat er t able near playa m ay reduce ET ( root s)
o Near borders of basin discharge t ow ard playa can be reduced ( st op flow t o cent er w here ET occurs)
• Ope r a t ion – increase rej ect ed recharge and prevent exist ing discharge: Q = ∆R + ∆D – ( + / - S ∆h/∆t )
• Ret ent ion dam s t o capt ure flood w at ers for recharge
1.72, Groundw at er Hydrology Lect ure Packet 6
