Groundwater Surface Water Interactions. · Regional Training Workshop on Integrating Groundwater...

Regional Training Workshop on Integrating Groundwater Management within River Basins, 15-18 January 2019, RCGW, KEWI, Nairobi, Kenya

Groundwater – Surface Water Interactions.


• Groundwater and Surface Water Interact continuously through the hydrologic cycle.• The hyporheic zone (stream bed) is the zone of most interaction.• Flows occur from surface water (streams, lakes etc.) to groundwater and vice-versa

depending on the hydraulic gradient.• Interactions occur not only as water volume fluxes but also as chemical and water quality

fluxes.• The direction, location and rate of these fluxes changes with time and season. They may

also be influenced by anthropogenic activities such as pumping, drainage or impoundment.

• Water resources management includes understanding both the nature, direction, location, timing and volume of these fluxes.

• A sound understanding of surface water – groundwater interactions lies at the core of catchment water management and optimum conjunctive use of the water resources.

Groundwater – Surface Water Interactions.


Key Differences between Surface Water Flows and Groundwater Flows.

• Location: • Surface flows occur in specific locations, streams and lakes, principally controlled by topography. • Groundwater flows tend to be distributed broadly through the sub-surface. Topography is less

important than formation hydraulic conductivity and hydraulic gradient.• Duration:

• Surface water flows are of short duration, usually a few weeks to a few months.• Groundwater flows take place over years, decades and millenia.

• Velocity:• Surface flows are rapid, of the scale of meters / sec. • Groundwater flows are slow, on the scale of meters / year or less.

• Evaporation:• Surface water is subject to high evaporation losses• Groundwater is largely protected from evaporation losses

• Quality:• Surface water is vulnerable to bacteriological contaminants and surface pollution. • Groundwater is protected from pollution but may be mineralized due to rock-water interactions.


Groundwater flow

• GW in continuous slow movement from recharge areas (usually upland areas) to dischargeareas (springs, baseflow, wetlands and coastal zones)

• Natural flow through an aquifer is usually at low velocities (e.g. 1 m/day is high!)

• The rate of flow and hence the turn-over time for groundwater is orders of magnitude slower than for surface water and hence groundwater requires a different management paradigm.


Baseflow is the groundwater component of stream flow. It varies depending on the climate, permeability of the basin and the topography amongst other factors.


Interactions between surface water and groundwater


• Streams gain water from inflow of groundwater through the riverbed

• Streams lose water to groundwater by outflow through the riverbed

• There can be a seasonal and spatial variation in whether a stream is gaining or losing




The effects of pumping on groundwater – surface water fluxes. Initially the groundwater feeds the stream, but in the last image, the flows are from the stream to the groundwater system.


Groundwater interact with wetlands

Similar to streams and lakes, wetlands can receive groundwater inflow, recharge groundwater, or do both:

• Wetlands that occupy depressions in the land surface have interactions similar to lakes and rivers

• Wetlands can be present on slopes; water table may intersect land surface, causing groundwater discharge, which permits the growth of wetland plants

• Riverside wetlands receive groundwater discharge, they have especially complex hydrological interactions and are subject to periodic water level changes


Reduction of groundwater discharge to springs and stream base – may affect

groundwater –dependent ecosystems, e.g. wetlands due to stress or loss of

resilience from inadequate water sources.

Groundwater Quantity: Over-exploitation

...lowering of groundwater

table may affect minor

wetlands


Some Consequences of GW abstraction

Excessive

pumping

When pumping is

further increased

Normal

consequences of

any groundwater

pumping


Source: GW-MATE

Groundwater Quantity: Over-exploitation….. Salinization


Historic data

When groundwater management starts to be implemented

Monitoring data

Baseline data (reference)


Management Challenges


GWM in Practice

• Identify critical points • Excessive drawdown with wells drying up

• Water quality deterioration such that it is unsuitable for original use

• Conflict between abstractors / users

• Uncontrolled waste dumping

• Decline of groundwater dependent ecosystems

• Reduced baseflow

• Declining springflow


Direct groundwater-surface interactions.

Groundwater use from shallow unconfined aquifers

• Groundwater use from shallow unconfined aquifers, delays the timing and reduces the amount of surface run-off in the rainy season and decreases baseflow in the dry season.

• Such baseflow may be of critical importance especially during the periods of low flow and in semi-arid climates.

• Baseflow provides provide perennial water to groundwater dependent ecosystems and the communities that survive from these resources

Pollution/contamination transfer between the two resources

• Interaction between surface water and groundwater can cause pollution to be transferred from one to the other.

• Groundwater pollution can more persistent for centuries thereby reducing water resources availability for generations to come.


Direct groundwater-surface interactions.

Groundwater recharge is impacted by surface water use.

✓Damming rivers and abstracting water reduces downstream flow for indirect groundwater recharge through riverbed infiltration.

✓This is often the major component of groundwater recharge in arid and semi-arid environments.

✓Irrigation excess and wastewater discharge (assumme regulated/permitted discharge) are also sources of groundwater recharge.


Examples of opportunities for conjunctive management

• Groundwater holds large volumes of water in storage, while surface water storage is moderate or small and often ephemeral. ✓A conjunctive management strategy could suggest to allocate

surface water resources before they run off or evaporate. (value of evaporation?).

✓In contrast groundwater use can be increased in the dry season to offset the shortfall from surface water.

✓Groundwater resources can provide a buffer in times of drought and water scarcity



• Managed aquifer recharge (MAR)

✓Can be done in identified suitable aquifers with surplus surface water during the wet season if there is excess flow.

✓Recharging aquifers in this way will not only provide additional dry season water resources but will also allow for natural purification of any bacterial contamination in the surface water during infiltration.

• Groundwater may be developed where demand is dispersed and moderate, while development of surface water may focus on large-scale demand and irrigation development.



• Balance upstream and downstream interests - demand and supply as influenced the interactions and connectivity between the surface and groundwater resources (surface flow transfers downstream are rapid)

• Financing of groundwater development and monitoring is a key area for flexibility.

✓In many instances, private and individual development of the resource takes place, particularly if the basin authority establishes a positive enabling environment such as, for example, subsidies for electricity or borehole drilling.

• Water resources, water managers are able to better balance the different competing needs in the catchment through integration of the entire suite of available water sources.


Exercise


EXERCISE

Purpose: To share experience of groundwater quantity problems.

Activity: Break into your groups

Each group to (in 1 hour):

a) Identify a common groundwater quantity problem in your country.

b) Discuss the nature and scale of the problem – is it anthropogenic or natural?

c) How is the problem being managed, and who is responsible for the

management?

d) What have been the aims of the management and how successful has it

been?

e) What would you change to improve management of the problem?


End


IssuesThe

hidden drought

300 m


IssuesThe groundwater

quality time-bomb


Why is Groundwater Management needed?

• Management and protection of groundwater has been seriously neglected, potentially endangering the resource.

• The sustainability of groundwater is linked to policy issues influencing water and land use, and represents one of the major global challenges in natural resource management.

• There is a need to integrate groundwater and surface water management to ensure better overall water management and allocation.

1

1111


Salinisation (or intrusion of saline waters) – another widespread impact of

excessive groundwater over-pumping.

Groundwater Quantity: Over-exploitation

➢ occurs due to up-coning of saline

water and mixing with fresh water,

giving rise to an aquifer salinisation.

➢ a major problem for many coastal

cities in some world’s river basins.


EXERCISE

Title: Water Quality Management

Issue: Mining impact on water quality in the West Rand: Calculation and interpretation

The West Rand area of Johannesburg is one of the largest gold producingregions in South Africa. The area is underlain by quartzites and dolomites. Someof the mines are closed in 1908’s and since 2002 acidic mine water is decantinginto the environment. Local farmers depend on the dolomitic aquifer for watersupply. The discharge of toxic-rich effluent from gold mines and the proximity ofslimes dams, tailings and rock dumps can also cause chemical and biologicaldamage to aquatic ecosystems through flooding, clogging, altering streams andwetlands and deposition of radioactive and toxic metals within the drainage ofthe karst system. The run-off from slimes dams enters the drainage network thatfeeds into the surface water, karst system and groundwater.


EXERCISE CONT

During the mining process, rocks which are situated far below the surface are brought to thesurface, where they are crushed and processed. Gold is extracted through chemical processingand the barren material is stored in slimes dams. The crushing and chemical processing exposeand mobilize pyrite (FeS2), a natural sulphur-rich component of the rock that is then exposed tothe atmosphere and water. The oxidized sulphates in combination with water and bacterialbreakdown produce sulphuric acid that in turn reacts with the rocks and soils to release andmobilize the metals. The acids and metals which are released are found in the rivers andgroundwater that is contaminated by the runoff from slimes dams, tailings, rock dumps and mineeffluent.


Task

Acid mine decant flows downstream into dolomitic terrain in the West Rand. Discharge

measurement was undertaken at six stations in two different months (February and August).

February is a rainy month, while August is dry. Station P1 exclusively contains acid mine decant

and the measured values change downstream.

Calculate the seepage rate/amount of acid mine water into dolomitic aquifer.

Give possible reasons for the loss, the impact and increase at some stations.

Recommend mitigation measures.


Acid mine decant in the West Rand, Johannesburg

Discha

rge

Februar

yAugust

Seepage in

Feb

Seepage

in Aug

Q

(m3/s)P1 2.214 1.075

Q

(m3/s)P2 1.2177 0.7

Q

(m3/s)P3 1.014 0.588

Q

(m3/s)P4 0.3 0.28

Q

(m3/s)P5 0.9 0.3

Q

(m3/s)P6 0.45 0.207

Total m3/s

Total m3/year

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Groundwater Surface Water Interactions. · Regional Training Workshop on Integrating Groundwater...

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