Water Quality 2
Lecture series outline Lecture 1a Water uses, water quality guidelines; physical, chemical and biological parameters Lecture 1b The origins of the constituents of water quality through the hydrological cycle Constituents of rain Influences of hydrological routing Acid rain and critical loads Lecture 2a Aquatic ecology habitats and niches Stratifcation Eutrophication Influences of reservoirs Lecture 2b Different land uses: Forestry Agriculture Urbanisation
Composition of water draining catchments Determined by: Rainfall Geology Hydrological characteristics Aquatic processes Terrestrial ecosystems Human interference
Dust and Sea spray
Chloride (mg/l) concentration affected by distance from the sea
Hydrological Characteristics Determine whether rainfall runs off or percolates into the ground. Susceptibility to Runoff determined by Steepness, roughness of slope Vegetation (interception) Soil texture. Ground porosity Drainage pattern Storm duration and intensity, direction of storm.
Hydrological Routing
The effect of rainfall Throughflow Initially brief increase in the concentration of TDS and SS. At end of rains, decrease in concentration weathered material washed out of the soil. Within Rivers Greater the flow, the greater the erosion rate and SS, which will encourages dissolved ions into suspension
Seasonal variation of TDS
Variation of TDS during storm flow
The effect of increased periods between rains Allows greater time for: Accumulation of organic substances and dusts on the vegetation and land surfaces, Build up of weathering and biological decay products Hence greater the concentration of the above in river waters when it does eventually rain.
Effect of increased runoff on stream waters The greater the amount of surface water flow the less the interaction with the local geology and soils and the less dissolved ions in solution.
The effect of increased Throughflow Dissolved ion concentrations and organic matter concentrations will reflect that of the soils Nutrient poor soils with little organic matter Waters likely to reflect the character of the rain Sands, Podzols, Oxilsols, Ultisols Acid waters Nutrient rich- water likely to reflect the character of the sediments: Clays, Inceptisols, Alfisols, Andosols, Mollisols: Slightly Acid or Alkaline waters
Acidity and soils As soils and rock weathers, major cations released and neutralise the acids in waters. ph is then raised Insufficient buffering Al released below ph 4.2, Silicic acid released below ph 4.0 - precipitates as clay when ph rises. Concentration of Silica is an indication of the weathering rate of the rocks. Phosphates can be released but are only soluble at near neutral ph Very strongly retained by soils, complexed by Fe and Al in acid soils and Ca in alkaline
The effects of increased Baseflow/ Groundwater flow The greater the ratio of groundwater from deep percolation to that of runoff, the: More likely stream water will reflect local geology rather than rain water Dominated by Na, Ca, Mg, Si, SO 4, Cl, HCO 3
Effect of local geology Major source of TDS. Effects of weathering Igneous rocks and metamorphic rocks resistant > sedimentary rocks. abundance of cations Mg>Ca>Na>K. Metal sulphates - mineral rich veins. Sedimentary rocks Less resistant more fractured Water acid soluble sulphates, carbonates, phosphates and Iron Limestones and Basic igneous rocks high Ca levels will impart a large levels of Ca to waters Sandstones and acid igneous rocks low Ca contents, little Ca in waters.
Some General Relationships Between Rock Types and Water Quality (Davisand DeWiest, 1966) Rock type Metamorphic and plutonic igneous Dolomite, marble Serpentine, dolomite, gabbro, and amphibolite Diorite, syenite Quartzite, marble, slate Granite, gneiss, rhyolite, and mica schist Gabbro, diorite, andesite, hornblende and gneiss Volcanic Sedimentary Shale Limestone Sandstone Water quality Almost always excellent; exceptions in arid regions and coastal areas; generally high silica content; low total dissolved solids Moderate to high hardness (calcium, magnesium) Hardness due more to magnesium than to calcium concentrations Dissolved silica 25-55 ppm Dissolved silica <30 ppm Slightly acid, lower total dissolved solids and hardness Slightly alkaline, higher total dissolved solids and hardness Good to excellent; exceptions near hot springs; tends to be "calcium-magnesium-bicarbonate" water, or when acidic "sodium-bicarbonate" with high silica content Variable; salinity increases with depth High amounts of iron and fluoride; ph 5.5-7.0 Low silica; high calcium and magnesium; ph >7.0 Variable; deep aquifers may yield soft water with high sodium and bicarbonate contents
Solubility dependant upon ph Weathering H + ions replace metal ions (Ca 2+, Na +, K + ) in rocks and hence resistance of different rock types Solubility (mmol/l)
ph independent reactions Ionically bonded minerals such as halite, and sulfate minerals, such as gypsum, have simple solubility relationships which are (almost) independent of ph. the reaction: NaCl= Na + + Cl - Describes the solubility of sodium chloride.
Types of Weathering Dissolution Congruent - Dissolution of chemicals into constituent anions and cations Incongruent e.g. Na Cl Na + + Cl - Formation of a new mineral as a result of water interaction e.g. Albite to Montmorillonite 3 NaAlSi 3 O 8 + Mg 2 + + 4H 2 O 2Na 0.5 Al 1.5 Mg 0.5 Si 4 O 10 (OH) 2 + 2Na + + H 4 SiO 4
Hydration Incorporation of water into the chemical compound (water of crystalization) e.g. formation of rust Fe 2 O 3 + 3H 2 O Fe 2 O 3.3H 2 O e.g. Gypsum CaSO 4 + 2H 2 O CaSO 4.2H 2 O Hydrolysis Reaction of water with chemical compounds Fe 2 (SO 4 ) 3 + 6H 2 O 2Fe(OH) 3 + 3H 2 SO 4 CaO + H 2 O Ca(OH) 2 Ca 2+ +2OH -
Carbonation Limestone CaCO 3 + H 2 CO 3 Ca 2+ 2 HCO 3 - Feldspars KAlSi 3 O 8 + H 2 CO 3 Al 2 Si 2 O 5 (OH) 4 + K 2 CO 3 + 4SiO 2
Vulnerable to pollution Less vulnerable to pollution Groundwater pollution.ppt
Zonation in aquifers The upper zone characterized by rain flushing solutes through the zone of aeration in to the zone of saturation typically water in the zone is HCO 3- rich (Soil CO 2 and calcite) and is low in TDS. The intermediate zone slower rate of groundwater flow and higher TDS. Sulfate becomes the dominant anion (Dissolved from minerals gypsum (CaSO 4. 2H 2 O) and anydrite (CaSO 4 ). The lower zone with very slow rates of groundwater migration. large amounts of soluble minerals- very little groundwater flushing has occurred. Typified by high Cl - concentrations and high TDS.
Chebotarev Sequence
Chebotarev Sequence Proceeds in groundwaters with Increasing distance Increasing residence time or age Mineral solubility Mg/ CaCO 3 < CaSO 4 < (NaCl, KCl) HCO - 3 HCO - 3 + SO 2-4 SO 2-4+ + HCO - 3 SO 2-4 + Cl - Cl - + SO 2-4 Cl -
Typical concentrations of elements in dilute oxygenated groundwater at ph 7 and their significance in terms of health and environmental protection.
Acid Rain Source Atmospheric Pollution, wet and dry deposition NOx and SO 2 burning of fossil fuels, fertilizers and smelting S compounds 65 million t /yr released NOx pollution increasing and currently accounts for half all NOx inputs Combined effect is to reduce rain, ph 2.1 has been recorded in Europe. Snow worsens the problem
Causes
Effects
Acid Flushes
External sources of Acid Rain External acid rain source % of total acid rain sources
ph scale in relation to ecosystems
Aquatic organisms tolerances to acidity
Interferes with biochemical pathways of organisms
Acid Rain Politics 1979, Convention and resolution on long range transboundary air pollution, 34 EU and N. American Countries BATNEEC used 1983, 21 European countries committed to reduce SO 2 emissions by 30% by 1993. UK not a signatory but did declare to meet this target before 2000
1988 EC large combustion plants directive Reduction of SO 2 1990 levels by 58% by 2003 Reduction of NOx and particulates by 40% by 1998. Each EU state had separate targets.
Critical Loads Load that can be applied to an ecosystem without resulting in deleterious effects Next set of atmospheric emissions negotiations concentrate on the critical loads approach http://www.environment-agency.gov.uk/yourenv/eff/pollution/acid_rain/?lang=_e http://www.nbu.ac.uk/negtap/finalreport.htm
Critical loads Calculation CL (A) = ANC w ANC le(crit) Where CL (A) = critical loads of acidity (Keq/ha/yr) ANC w = Acid neutralising capacity produced by weathering (Keq/ha/yr) ANC le = critical leaching of ANC (Keq/ha/yr) ANC = sum of cations mainly Ca 2+
Exceedence of critical loads in 1995-1997 covered 71% of sensitive ecosystems in the UK and is expected to be 46% in 2010 NOx deposition will be the main problem in 2010 Some evidence of recovery in UK upland soils and rivers
The effect of Organisms, vegetation and soils
The effect of Organisms, vegetation and soils Vegetation organic compounds from leaves roots and decaying material. Uptake by plants of nutrients Storage of minerals and nutrients in plant biomass, will have a seasonal effect on the concentrations of some nutrients.. Entrapment of atmospheric dusts Fixation of N Binding of sediments The litter layer effectively protects the underlying soils from rainfall splash and erosion. Mediating Temperature Organisms Faeces, urine and other excretions from animals Anaerobic bacterial activity may reduce some metals and make them more likely to leach into water bodies.
Ecological sucession TDS (log scale) Establishment -biomass of catchment builds, retention of elements begins Disturbances Man, fire, earthquakes, volcanoes, hurricanes. A pulse of dissolved components and sediments Time Climax - as much of an element that enters the system through rainfall or weathering leaves the catchment in stream water.
Hubbard Brook experiment. Catchment; granite with some shales Lots of individual catchments considered to be impermeable Catchments covered in natural broadleaf forest Nutrient loss via streams
Reference watershed http://lternet.edu/sites/hbr/ http://www.hubbardbrook.org/
Undisturbed forested catchments Weathering supplies most of the Ca, Mg, Na, K and P. Rain and snow bring large amounts of Na, N, and S. Most Ca, Mg, Na and S is washed out in dissolved formplants have little restraining effect on these elements. K and N are mostly retained by the ecosystem, though 20-30% are lost in stream water. P is very strongly held in the ecosystem, less than 1% lost in stream water. Comparing the rain water input to the drainage water there is a net gain of Na, K, Mg and Ca. but a loss of N, P and S in stream water.
Comparison of forest and moorland
Other Vegetation effects on water quality Some vegetation may produce pollutants e.g. Bracken- spores are a stomach irritant Conifers - phenols are leached which leave an unpleasant taste Peaty moors produce coloured waters which may require treatment.