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Unit 05 : Advanced Hydrogeology

Groundwater Resources

Groundwater as a Resource

• Large Regional Flow Systems

• Well Yield Analysis

• Artificial Recharge

• Conjunctive Use

• Aquifer Management

Potential of Groundwater Resources

• (^) Groundwater resources in many countries are coming under

increasing threat from growing demands, wasteful use, and

contamination.

• Surface water resources are particularly vulnerable to

pollution, and are often limited in magnitude, particularly in

arid regions.

• (^) Groundwater resources are hidden and often poorly

understood, but they are widespread, relatively easy to

protect from contamination, and their development potential is

great.

• Shallow groundwater in particular is relatively easy to access,

and suitable for small scale development for domestic,

livestock, and irrigation use in less developed countries.

Groundwater Replenishment

• (^) The replenishment or recharge of shallow aquifers is of particular interest, as the annual recharge represents an upper limit of the quantity of water which can be abstracted without “mining” groundwater.
• (^) In most situations, however, natural recharge to an aquifer emerges somewhere as natural discharge, sustaining stream flows and keeping wetlands wet.
• (^) Therefore only a proportion of recharge can be abstracted for consumptive uses such as irrigation, in which used water is not returned to the aquifer.

Safe Yield

• Management of groundwater systems requires

some kind of yield analysis to determine how

much groundwater is available.

• The term “safe yield”, coined by Lee(1915), is

used to denote the sustainable maximum rate

at which water can be withdrawn without

dangerous depletion of storage.

• Lee’s definition is conceptually sound but

lacks any guidance on what might constitute

“dangerous depletion”.

Conkling and Banks

• Conkling (1946) was more specific and much

more controversial:

• “Safe yield”, defined by Conkling and modified

by Banks(1953), is an annual extraction rate

that does not:

1. Exceed the average annual recharge;
2. Lower the water table so that pumping is uneconomic;
3. Lower gradients so as to admit intrusion of water of undesirable quality;
4. Fail to protect existing water rights.

ULARA

• (^) The Upper Los Angeles River Area (ULARA) is a useful example of the application of safe yield concepts.
• (^) ULARA is bounded by mountain ranges and hills and forms an identifiable basin.
• Groundwater is extracted from a thick and productive valley-fill aquifer in an arid region with limited recharge (precipitation = 356 mm/y).
• (^) As population grew and water demands increased, natural discharge to streams and rivers was eliminated and well-hydrographs demonstrated a 30-60 m decline in storage between 1930 and 1968.

Recovery

• (^) In 1968, groundwater withdrawals were limited by court

action to 350,000 m^3 /d

• (^) The rate prior to the court action was >500,000 m^3 /d
• (^) Hydrographs subsequently levelled out and recovered by the

mid 1990’s

• (^) The safe yield of the San Fernando Basin (the major basin in

ULARA) was estimated at just under 150,000 m^3 /d based on

recharge from precipitation.

• (^) Water is also imported into the basin to supplement recharge

and this amounts to about 150,000 m^3 /d so the total recharge

balances the total withdrawals (the withdrawals for the San

Fernando Basin alone are about 300,000 m^3 /d)

Groundwater Basin Management

• GBM is defined as planned use of basin

yield, storage space, transmission

capabilities and water in storage, including:

1. Protection of natural recharge and use of artificial recharge;
2. Planned variation in the amount and location of pumping over time;
3. Conjunctive use of groundwater and surface water sources;
4. Protection and maintenance of groundwater quality.

Management Tools

• Basin management requires a well-defined

limit on the quantity of water that can be

pumped which is largely based on the safe

yield concept.

• Artificial recharge encompasses a range of

engineered activities to supplement natural

inflows, and

• Conjunctive use involves the combination of

surface water and groundwater sources to

meet a single demand.

Early Artificial Recharge I

• (^) Early interest in artificial recharge focused on the use of drainage (recharge) wells to reclaim wetlands for agriculture. Many of the drainage wells failed due to clogging by sediment suspended in the drainage water.
• (^) Drainage wells that recharged storm runoff and sewage to the Floridian aquifer, consisting of highly porous and permeable limestone, were more successful.
• (^) Drainage wells tapping highly fractured basalt aquifers were also successful including use of a pit and wells to drain surface water into basalts in the Snake River Plain of Idaho.

Early Artificial Recharge II

• (^) Significant interest developed during the 1930s, in the use of

artificial recharge to conserve or enhance groundwater

storage.

• In California, artificial recharge of alluvial aquifers with storm

runoff by use of spreading basins was a widespread

practice by the 1930s.

• (^) In New York, water levels in a significant area of western

Long Island had been drawn down below sea level by the

early 1930’s due to groundwater pumping, much of it for air

conditioning.

• (^) Legislation passed in 1933 required that groundwater

pumped for air conditioning be recharged, either by well

injection or through spreading basins.

Water Spreading

• (^) High permeability materials (sands and gravels) can give recharge rates from 0.5 to 15 m^3 /d/m^2.
• (^) Recharge rates reduce over time as the effective head gradient reduces. - (^) Water spreading involves diversion of surface water to topographic lows such as abandoned pits or detention ponds and reservoirs. - River valley sands and gravels are utilized by releasing stored storm-water to infiltrate slowly through streambeds. - (^) Water traps are employed to increase infiltration in streambeds. The traps are earthen dams of variable height, usually 1 m to 3 m, that are constructed of locally available materials.

Recharge Lagoons and Trenches

• (^) Unless particulates are controlled by sedimentation in a separate basin, clogging can become a serious problem. - (^) Cutwaters are excavations of variable dimensions, used as reservoirs, built in low-lying areas. Their primary objective is the harvesting and storage of surface waters. - Recharge lagoons are custom- built impoundments in the recharge areas of aquifers. - (^) Wastewater recharge lagoons can be used instead of clean water and improvements in both bacterial and chemical quality are possible in the recharge process (Coliforms, BOD, COD, TOC, NH 4 +^ and NO 3 -^ ).