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Water Occurrence in Arid Areas Robert Read AIG Conference 2015 After English et al. 2012 Not much moves without underground water Towns Tourism Pastoral Mining Characteristics of the Arid Zone Erratic rainfall For stations with long records wettest 5 consecutive years 9 to 10 times as much rainfall as driest 5 consecutive years. Mostly low relief Relatively well vegetated. Plants that are very effective at scavenging water. Aquifers Widespread, but not always where needed. Variable water quality. Recharge Average low to very low . Erratic, often in La Ninya years Characteristic “saw-tooth” hydrographs RN 5586 548 500 400 300 546 200 545 100 544 0 1/3/65 12/17/75 11/29/86 11/11/97 10/24/08 Monthly r ainfall mm WL in m AHD 547 Average water levels deeper than in humid areas. Overall dissolved solids are higher, ranging up to hypersaline in salt lake areas. Ti-Tree Basin and a catchment in N Oman Wadi Zarub Ti-Tree Basin and a catchment in N Oman Ti-Tree Basin and a catchment in N Oman Recharge and Vegetation Plants may be able to take water from about 20 m depth. Mulga, Acacia aneura Pre-dawn leaf potentials as low as 7 Mpa. Taking water from silty soils at shallow depth. Water use declines in dry periods. (95% decline in Ti-Tree Basin, April to Nov 2007.) Evapotranspiration 0.5 to 0.2 mm/d Bloodwood, Corymbia opaca Like mulga perseveres through drought. Water used does not decline much in dry periods. Pre-dawn leaf potential 2 Mpa. Appears to be taking water from about 6 m, deep in the unsaturated zone. Water use remains near constant in dry periods Eucalyptus camaldulensis Grows along creek lines Pre-dawn leaf potential as low as -1.2 Mpa Evapotranspiration 2.4 mm/day How does any recharge water get down? How does any recharge water get down? Recharge is probably largely due to macropermeability. Burrows (e.g. termites) in the top few m, fractures and fissures below that. Termites, lizards, scorpions, small mammals all burrow. Kay Kessing “Spinifex Times” Oral account of a well-sinker finding a “native rat” burrow at 6 m in weathered crystalline rock. Some small caves in calcrete reach the water table. Effect of geology Soil over silty sand, water may get down through macropores, but will move very slowly below. Soil over fissured calcrete, more infiltrating water will get down to the water table Ti-Tree Basin Lower salinity water not under creeks, but under areas of shallow calcrete. • This catchment discharges from a permanent spring, easy to estimate recharge. Rapid direct recharge of about of 25 mm/y, about 10% of rainfall. Aquifer types Broadly the aquifers in the arid zone can be classified into four types, though the classifications overlap: Major basins, with mostly primary porosity. Palaeo-channels and epeirogenic basins Fractured and weathered sedimentary rocks, usually older than the major basins. Fractured and weathered crystalline rocks. Palaeovalleys Palaeovalleys range in age from Quaternary (Alice Springs Town Basin) to possibly Permian. Aquifers in them include coarse fluvial sands and fissured silcrete and calcrete. They may be evident in outcrop and imagery or completely obscured by dune fields. The map above is the major palaeovalleys, there are many more minor ones down to channels 50 m wide with bores that provide small stock supplies. Pipalyatjara Palaeovalley, far NW of SA, clearly marked by calcrete. Eastern end of the Mann Range, 125 km from previous. Palaeovalley found by blind chance. Older Sedimentary Basins Generally the rocks in older (say mid Palaeozoic) sedimentary basins will have low primary permeability. They are good aquifers when permeability and porosity have been enhanced by weathering. Aquifers are localised where topography and structural features have allowed the greatest weathering. Yields may range from low to 100 L/s for the Alice Springs town supply. Weathering history is very important for the formation of aquifers. Downhole optical scan of bore at Roe Creek, showing large fissure. Limestones, dolomites vary hugely along strike depending on solution fissures. Example Siltstone with magnesite pellets. In some locations magnesite had been dissolved out leaving a very permeable rock. Fractured crystalline rocks These are the most frustrating. The deep water tables result in shallow fractures that might yield water in more humid areas being dry. In general, yields are only enough for pastoral and small communities. On rare occasions aquifers in highly weathered rocks yield up to 90 L/s in the short term. Evaluating fractured rock aquifers This is a nightmare. Pump test evaluation is difficult. Some tests show Theis or Jacob straight line responses. Some one dimensional flow responses. Drawdown plotted against arithmetic square root of time is close to a straight line. Fractional dimensions (Barker, 1988) often appear to fit. Extrapolating to the long term is problematic. For single hole tests square root of time plots (1 dimensional flow) will generally give reasonable estimates for about 6 months to 2 years. Only experience will show long term yields. Engawala Community, population of 140 supplied by production bores in a fault zone in gneiss with generally low permeability. Despite the apparently limited storage and the presence of saline water nearby the supply has been maintained for 40 years. Engawala Engawala Yuendumu Community of about 800 people, 300 km NW of Alice Springs. Designated growth centre. Average age 25. Yuendumu Yuendumu Historically developed on small fracture aquifers in crystalline rock , long since outgrown. Current water supply is from fractured Devonian and Carboniferous sandstones in the Ngalia Basin. Recharge is very small. Aquifers have progressively dewatered. Will require development of further aquifers in the future. Yuendumu Yuendumu Roe Creek Borefield, Alice Springs After various attempts at digital modelling best results by plotting water level against cumulative extraction. Water Quality Even in areas where salinity is low, water may not be suitable for human consumption because of high nitrates and or fluorides. Nitrates Nitrates are largely associated with direct recharge, and appear to originate from termites which are very common in the arid zone. Surfer plot of all nitrate values in the NT. Nitrates stop at the edge of the Barkly blacksoil where no termites live. Fluorides Fluorides are common in hot areas throughout the world, especially in crystalline rocks with fluoride containing minerals. The arid areas offer enough challenges to keep many more generations of hydrogeologists entertained.