Basin Development Concept of Safe Yield - groundwater systems need to be thought of as 3-D systems, but we often simplify for calculations - groundwater flow occurs in groundwater basin - safe yield (or optimal yield) Amount of water that can be withdrawn from a groundwater basin annually without producing an “undesired” result - withdrawl in excess of safe yield is known as overdraft - undesired results - mainly depletion of reserves - also intrusion of water of undesirable quality - contravention of water rights - deterioration of economic advantages of pumping - excessive depletion of streamflow by induced infiltration and subsidence

- safe yield has to be balanced against socioeconomic demand for the water - at some times it may be necessary to “mine” groundwater to depletion - other cases may call for complete conservation - some have suggested that safe yield is the annual extraction of groundwater that does not exceed annual recharge - not correct - removal of water changes regime/recharge

Artificial Recharge - in many large populated regions of N.A. safe yield has been met or exceeded and - into management of water resources - in many cases involves movement of water from one area to the other, or holding water in good times for use in bad times - groundwater often good place for storage because of low ET losses - any artificial transfer of surface water into groundwater system is artificial recharge - most common method is the construction of spreading basins - e.g. construction of dikes in channels - growth of groundwater mound beneath spreading basin - area and extent of the mound depends on: 1. size/shape of recharging basin 2. Duration and rate of recharge 3. Nature of subsurface formations 4. saturated and unsaturated properties of materials

- Many models developed but reality of difficult to model. - even if water levels kept constant in spreading ponds R usually declines with time because of silting in the pond - also microbial organisms clog pores over time - often best to have dry periods that kills mirobes, and manually till pond bottom to reopen system - injection wells have also been used for recharge - injection of storm runoff - cooling water change of flow properties to ward off salt-water intrusion

Basin Development continued... Seawater Intrusion (Saline water encroachment) - problem in coastal areas - downward sloping nature of coastal plain means that flow is generally from land to sea - because of nature of coastal zone deposits, usually interfingering of permeable and impermeable layers -salt water freshwater boundary is dependant on the amount of freshwater flowing out of an aquifer - anything that changes the amount of water flowing from aquifer, changes the boundary - human action can change coastal head patterns to cause enroachment of saline water into previously freshwater aquifers

First: How do fresh and saline groundwater interact? - up to now most discussion/equations assume freshwater - as you move from fresh to saline there will be a zone of dispersion - in the zone salinity will generally increase in a linear fashion with distance - zone can be sharp (short distance), but may grow due to e.g. tidal forcing - a sharp boundary is assumed for the calculations that follow

- nature of coastal fresh/saline contact is predictedby the Ghyben-Herzberg principal where z(x,y) is depth of saltwater/fresh interface below sea level at location x,y h(x,y) is the height of the water table a.s.l. at the same point ρ w is the density of fresh water ρ s is the density of the saline water - this assumes both water bodies are static

- for relatively fresh water and standard sea water salinities this works into a general rule for the more realistic situation where there is freshwater flowing into the ocean: where q’ is discharge from the aquifer at the coastline per unit width - this is the Dupuit-Gyben-Hertzberg model

- this model does not treat the boundary at the coastal surface very well so has been modified (Glover modification) - this allows z to still have a value when x=0 - using this relation can calculate the width of the outflow face:

magnitude of Submarine Groundwater discharge (SGWD) has been undergoing some radical re-evaluation in the last few years - previously though to be 0.01 to 10% of surface runoff to oceans, based mainly on point measurements and modelling - study in 1996 used tracers naturally enriched in groundwater, e.g. 226 Ra - build up in coastal areas can tell of magnitude of SGWD - conclusion of study was that at least in southeastern U.S. SGWD is comparable to surface discharge into oceans Passive Saline water encroachment - occurs where some water is being diverted from the fresh aquifer - hydraulic gradient still seaward - boundary slowly moves landward because of potential change - happening today in many coastal aquifers - very slow process...100's of years for significant movement

Active Saline water encroachment - reversal of hydraulic gradient - caused by intense pumping - freshwater now flows away from interface - interface moves until is at low point of gradient...well - both types of encroachment can occur in connate aquifers as well Question: What is connate water?

Abatement of Seawater Intrusion 1. Reduction or rearrangement of the pattern of groundwater pumping 2. Artificial recharge of the intruded aquifer from spreading basins or injection wells 3. Development of pumping trough adjacent to the coast 4. Development of a freshwater ridge by injection wells parallel coast 5. Construction of artificial subsurface barrier - only #1 has been found to be economically viable Another option: 6. Lower sea level

Oceanic Islands -Freshwater aquifer surrounded on all sides - lens of freshwater floating on salty -Solution for an infinite strip island gives water table profile across the island - w is recharge rate, width is 2a, h is water table head at any distance x from the shoreline See example problem on page 336 for detailed steps on solving interface depth Circular island with an island radius of R and r is equivalent to x in previous equation but is distance away from the center of the island

Tidal effects -Water tables near the coast fluctuate with the tide - amplitude of effect is highest at the coast and diminishes inland x Where t o is time to go from one extreme to the other Is time lag of tidal response inland