Module 2: Characterization of Groundwater Systems

1. Module 2: Characterization of Groundwater Systems

2. INTRODUCTION Groundwater differs from surface water through its physical and chemical environment Among aquifers there are huge differences with respect to geological environments Resulting in their capacities to store water and to transmit water flow as well. Hence the availability of groundwater will depend on hydrogeological setting characterized by hydraulic parameters Groundwater management requires reliable aquifer characterization.

3. Groundwater…the “hidden” resource • Underlying resource, stored in underground reservoir (rocks) and transmitted through interconnected spaces • Aquifers have huge differences with respect to their hydrogeological setting. • GW management based on a good understandingof aquifer system.

4. What is Groundwater?

5. Groundwater

6. What is groundwater? • Definition of Groundwater/Hydrogeology • The rainfall that soaks into the ground and moves downwards into spaces and cracks in the rocks below the ground surface becomes groundwater. • The study of groundwater is called hydrogeology.

7. Aquifers main functions • Storage capacity (storage coefficient or specific yield) • Transfer capacity (transmissivity) • Physical and chemical interaction capacity (reservoir-rock vs GW)

8. Hydrogeological diversity • Unconsolidated rocks: • Pore spaces • Large storage • Consolidated rocks: • Fractures • Small storage • Consolidated rocks: • Karsts (enlarged fractures) • Large storage

9. Basement and Sedimentary Aquifers

10. Why Groundwater • Why Groundwater? • A readily available source at point needed • Its relative low costs compared to surface water • Availability in most areas • Potable without treatment • Employs low cost technologies • The frequent drought problems enforce the use of groundwater source as many small intermittent rivers and streams dry out during the dry season

11. SS Africa Hydrogeological setting • Minoraquifers • Critical issues: • availability • reliability AfterMacDonald et al, 2005

12. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? • Groundwater characteristics: • Groundwater occurs in most geological formations because nearly all rocks possess openings (pores or voids, or fractured). • All aquifers have two fundamental characteristics: • A capacity for groundwater storage (Porosity ) and; • A capacity for groundwater flow (Permeability). • An aquifer is a geological formation capable of yielding useful groundwater supplies to wells and springs.

13. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? • Aquifer types: • unconfined (free surface), called water-table aquifer or phreatic aquifer; • confined (under pressure) aquifers are bounded by impervious or semipervious layers. (Schmoll, O.,et al. – 2006.)

14. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? • Hydrogeological environments: • The availability of groundwater depends primarily on the geological environment • Aquifer unit storage capacity (storativity) varies extremely between: • Unconsolidated granular sediments, and; • Highly-consolidated fractured rocks. • Groundwater flow potential (transmissivity) depends on aquifer saturated thickness and types of aquifer layers (permeability)

15. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? • Hydrogeological environments: • Ancient crystalline and metamorphic: highly fractured or weathered: • permeability is low, the storage capacity as well. • GW potential moderate (0.1 – 1 l/s), since aquifer is usually of minor extension. • Targets: fractures at the base of the deep weathered zone, and the vertical fracture zones.

16. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? Groundwater occurrence in weathered basement rocks (MacDonald et al., 2005)

17. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? • Hydrogeological environments: • Consolidated sedimentary aquifers: large groundwater storage and major regional extension, → different aquifers types: • Sandstones: moderate to high potential (1 – 20 l/s); • Targeting for abstraction: coarse porous or fractured sandstones • Mudstone and shale: fractured or interbedded with sandstone layers; • its potential is low (0 – 0.5 l/s) with minor regional flow; • Targets: hard fractured mudstone or thin sandstone layers • Limestones: slightly soluble in rainwater → fractures can be enlarged to form karsts (developed conduits and fracture systems) • Its potential is moderate to high (1 – 100 l/s).

18. Different hydrogeological environments

19. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? • Hydrogeological environments: • Unconsolidated sedimentary aquifers: loose materials, • GW stored and transmitted through pore spaces, not fractures. • Large to very large storage capacity and usually huge regional extension. Distinguished major hydrogeological units are: • Major alluvial and coastal basins: sands, gravels and clays : • High potential (1- 40 l/s), Sands and gravels are targeted layers. • Small riverside deposits: mixed alluvium (cobbles, gravels, sands, silts clays), and others deposits like coastal dunes (sandy): • potential moderate (1 – 20 l/s), sandy/gravel deposits targeted . • Valley deposit in mountain: poorly sorted rock fragments, sands, gravels, volcanic materials: • Moderate to high potential: 1 – 10 l/s, Stables areas of sands and gravels are interesting targets.

20. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? Groundwater occurrence in riverside deposits (MacDonald et al., 2005)

21. WHERE DOES GW OCCUR, IN WHAT ENVIRONMENT? Summary of key properties of the most widely-occurring aquifer types (GW Mate2, 2006)

22. HOW GROUNDWATER DOES FLOW? • Groundwater moves slowly through interconnected pore/fracture spaces of aquifers materials. • GW in continuous slow movement from recharge areas (usually upland areas) discharge areas (springs, baseflow, wetlands and coastal zones). • The flow of groundwater through an aquifer is governed by Darcy’s Law. • Aquifer storage transforms highly variable natural recharge regimes into more stable natural discharge regimes. • It also results in groundwater residence times, usually counted in decades, centuries, even millennia: • Large volumes of so-called ‘fossil groundwater’ still being held in storage. • Residence time will depend also on aquifer geologic formations that bear groundwater

23. HOW GROUNDWATER DOES FLOW? • Where aquifers dip beneath much less permeable strata, their groundwater becomes confined (to varying degrees) by overlying layers. • This results in a corresponding degree of isolation from the immediately overlying land surface, but not from the groundwater system as a whole. • Drawdown induced by pumping from the confined section of an aquifer is often rapidly propagated to the unconfined section. • In various hydrogeological settings, shallow unconfined and deep confined aquifer layers can be superimposed → leakage downwards and upwards between layers according to local conditions.

24. HOW GROUNDWATER DOES FLOW? Typical groundwater flow regime and residence times of major aquifers under semi-arid climatic regimes (after GW-Mate2, 2006)

25. Groundwater flow • GW in continuous slow movement from recharge areas (usually upland areas) discharge areas (springs, baseflow, wetlands and coastal zones)

26. Relationship between groundwater and surface water • Diagnosing the relationship surface water / underlying aquifer is an important component of groundwater system characterization. • It is important to distinguish between: • Streams and rivers on which an aquifer is dependent as a significant source of its overall recharge • Rivers that in turn depend significantly on aquifer discharge to sustain their dry-weather flow. • It should be noted that in some cases rivers may fluctuate seasonally between two of the conditions depicted.

27. Relationship between groundwater and surface water Spectrum of possible relationships between surface watercourses and underlying groundwater systems (after GW-Mate2, 2006)

28. Relationship between groundwater and surface water • Changes to surface water bodies in response to groundwater development can cause significant consequences: • Long term reductions in stream flow can affect vegetation along streams • Changes in the flow direction to/from streams may affect temperature, oxygen levels, and nutrient concentrations in the stream (affect aquatic life) • In gaining and in losing streams, affect hyporheic zones (active sites for aquatic life)

29. Relationship between groundwater and surface water • Lowering of lakediminish lakefront aesthetics, and shoreline structures such as docks, may alter the natural fluxes to lakes of key constituents (nutrients and dissolved oxygen) • Amplitude and frequency of water-level fluctuationsaffect wetland characteristics (type of vegetation, nutrient cycling, type of invertebrates, fish, and bird species) • Lead to reductions in spring flow, changes of springs from perennial to ephemeral, or elimination of springs altogether • Plant and wildlife communities adapted to coastal areas can be affected by changes in the flow and quality of groundwater discharges

30. Groundwater recharge • Recharge may result from: • Precipitation that percolates through unsaturated zone to water table • Losses of surface water bodies (steams, lakes, wetlands) • Contemporary aquifer recharge rates, a fundamental consideration in the sustainability of groundwater resource development. • Understanding aquifer recharge mechanisms and their linkages with land-use is essential for integrated water resources management. • The quantification of natural recharge rates, subject to significant methodological difficulties, data deficiencies and resultant uncertainties because of: • Wide spatial and temporal variability of rainfall and runoff events, and; • Widespread lateral variation in soil profiles and hydrogeological conditions.

31. Groundwater recharge • Recharge rates vary with: • river flow diversion or control, • modifications to surface water irrigation, • changes in natural vegetation or crop type in recharge areas, • reduction in leakage from urban water-supply networks and in-situ wastewater percolation, • lowering of water-table, etc. • A number of generic observations can be made on aquifer recharge processes: • Areas of increasing aridity will have a much lower rate and frequency of downward flux to the water table, • Indirect recharge from surface runoff and incidental artificial recharge arising from human activity is generally becoming progressively significant than rainfall direct recharge.

32. Assessment of groundwater resources • To understand groundwater systems and how they respond to external and internal changes, →need to be able to assess groundwater movement: • local scale (seepage phenomena, flow near groundwater wells), • regional scale with recharge/discharge of aquifer systems, interaction of different aquifers, interaction with surface water bodies • Many difficulties encountered in groundwater assessment due to various reasons: • groundwater cannot be readily observed, it is hidden resource • usually groundwater occurs in large, and complex aquifer systems • aquifers have high spatial variability of its characteristics

33. Safe yield • Safe yield: the attainment and maintenance of a long-term balance between the amount of annual withdrawal and the amount of annual recharge. • This allows water users to abstract groundwater no more than it is replenished naturally. • So-called ‘safe yield’ is clearly bounded by the current long-term average rate of aquifer recharge, although should also consider: • value judgements about the importance of maintaining some of the natural discharges from the aquifer system • consideration of consumptive use and catchment export, as opposed to local non-consumptive uses which result in the local generation of an effluent.

34. Critical issues of GW characterization • Recharge rate quantification • Recharge area vs land-use (GW protection) • Interactions (quantity/ • quality) with surface water bodies • Impacts of GW pumping • Interbasin/interboundary aquifer • Aquifer system water bal • to check our understanding • Modellingapproach: • to predict implication of impacts

35. Difficulties and uncertainties • Groundwater cannot be readily observed; • GW may occur in large, and complex aquifer systems; • Aquifers have high spatial variability of its characteristics.

36. The neededinteraction to providesuitable GWR information Policy level Other stakeholders Demand side Water provision and management Policies Strategies Regulatory framework Sustainabledevelopment GW protection Groundwater Understanding of GW system Information on unit system Knowledge on aquifer properties, and Technical solutions

37. Assessment of groundwater resources • Different ways to assess groundwater: • Observation of groundwater levels • Pumping tests, to test the response of groundwater abstraction • Hydrogeological investigations to built a first concept on groundwater resource; • Geophysical surveys to find the groundwater resource, to sit borehole • GW budgeting, modelling for better resource development planning • Groundwater Quality need to be assessed to protect GW resources: • Salinity monitoring, Other field measurements of water quality parameters • Analysis of groundwater samples (in field or in laboratory)

38. GW location and storage • GW location, a big issue in complex hydrogeological setting • Available GWR for current and future use, for differentuses…sustainably! • Criticalelements: safeyield , overexploitation (negative impacts costs)

39. Recharge and recharge areas • Recharge ensuresrenewability of GW storage, input to the system! • Recharge rate estimate, a critical issue for sustainability of GW development • Recharge area map, for land-use planning and access to

40. GW discharge • SW and GW (in many cases) hydraulically connected…all too often ignored in water management considerations and policies • Useful management information? maps of high risk areas of extensive GW exploitation • Trend of characteristics as impacted by land-use/change, climatechange/variability, water use

41. GW naturalquality occurrence • Areas and levels (vertical) of naturally-occurring hazards • Geological settings potentially bearing affected groundwater • Maps of occurrence of groundwater concentration for harmful elements • Locate alternative (ground) water resources to ensure reliable water supply

42. Thank You!

43. EXERCISE • Characterisation of groundwater systems • Purpose: To appreciate the link between understanding groundwater systems and strategies for management • Duration: 30 Minutes • Activity: In 4 groups, participants discuss how the knowledge of specific characteristics of aquifers can improve the management of groundwater (Local aquifers and international boundaries aquifers. • Report Back: Each group presents a table with their identified aquifer characteristics and how each one improves groundwater management.