Env Geochem

• the hydrosphere - all of the water on earth. 70% of earth is covered by oceans. total water budget is ~ 1.386 billion km^3. total amount of water doesn't change over time but the proportions of water held in each reservoir can, and have, changed. majority is saline, only 2.5% is freshwater. ~ 3/4 of this freshwater is currently locked away in the planets icecaps.
• the hydrological cycle water vapour in the atmosphere condenses and may fall as rain. most of this rainfall will reach the surface of the earth but some will be intercepted by vegetation or buildings and undergo evaporation
• rainfall that reaches the earth's surface may:

  remain at the surface in ponds and puddles and undergo evaporation

  flow across the surface into rivers, lakes or the oceans where again it may be evaporated or seep into groundwater,

  or,infiltrate the surface to become soil moisture where again it may evaporate, flow through the system to seep into lakes or rivers or percolate into the groundwater

  groundwater is eventually returned to the surface by capillary movement
• rainfall is the main input of water to the earth's surface - it controls soil moisture, groundwater recharge or river flow. water vapour has a mean residence time of 10 days in the atmosphere, but this hides large variations. waterhigh up in the stratosphere may stay there for >10 years, whereas water in a thundercloud may only remain for < 1hr before it's returned to the earth's surface
• mechanisms that generate rainfall there are 3:

  orographic: mechanical lifting of moist air over e.g. mountain ranges, or islands

  cyclonic: frontal (moist air forced up over cold air) and non-frontal (convergence and uplift of air in a low pressure region)

  convectional: heating of ground surface causes up-currents of thermally unstable air
• rainfall chemistry as water condenses and falls through the atmosphere it will rapidly dissolve gases and particles, 'scrubbing' or 'cleaning' the atmospherewater droplets forming in the atmosphere are 'pure' but their chemistry changes rapidly after formation due to this cleaning of the atmospherethe duration of a rainfall event can influence the composition of the rainfall. the initial rain that falls may contain more dissolved material than rain that falls towards the end of the rainfall event.water condensing over the oceans or in coastal regions will dissolve salt aerosols and have the co...
• to understand the data and identify sources of additional ions in rainwater you need to... first subtract the contribution from seawater:identify a 'conservatuve' ion e.g. Cl- that shows litter fractionation, i.e. is unlikely to have changed. we can assume that the chloride in the rainfall has all come from the sewater as its not typically derived from terrestrial sources and is an uncommon pollutant.for each other ion calculate the ratio to Cl- in seawater: e.g. for Na+ Na concentration in seawater = 485 mmol L-1 Cl concentration in seawater = 566 mol L-1 Na/Cl = 0.86 The concentration of C...
• geological controls on water chemistry rainfall acquires a chemical signature as it falls through the atmosphere and reacts with particles and aerosols, once it reaches the ground it will further interact with the soils and rocks it contacts as it percolates into the ground to become groundwater. geology, geomorphology and climate will all influence the chemistry of surface and ground waters, the majority of the dissolved ions present in river waters is derived from weathering of soil minerals and rocks. also affected by reaction with gases and through mixing with other waters (extra card)
• water chemistry is affected by reactions with gases and through mixing with other waters In mountainous regions where rainfall will run-off the slopes rapidly there is only a short time available for the water to interact with the rock. Rivers in such regions tend to be fast flowing and turbid, able to carry particles along in their flow. In flatter landscapes rainwater has time to infiltrate the soils and react with the mineral particles. River flow in such landscapes is slower and particles tend to settle out of the flow rather than be carried within it. In more unusual situations suc...
• effects of chalk, limestone, and impervious rocks on water chemistry In general; Chalk and limestone catchmentsclear hard waters.water spends a long time in the aquifer (dissolves Ca2+, Mg2+ etc.).rivers tend to have constant flow as they are fed from underground resources. Impervious rocks e.g. graniteturbid soft waters.mostly fed by run off from highlands.carry silt and sediment. Water is constantly moving so other processes also affect solute composition e.g. velocity of the water, residence time.diffusion, the rate at which reactants can diffuse together.
• how to determine the dominant processes controlling water chemistry at a given location? contributions from both rainfall and geology. easier if you can identify and remove the rainfall contribution. first step is to establish the extent to which evaporation has increased the concentrations of ions in your water, again a conservatice tracer ion is needed. Chloride is present in rainfall as it is derived from sea spray but has few other sources and does not readily react - therefore it is typically chosen as the best ion to use to calculate an evaporation factor: For example: Concentration o...
• general findings from dominant processes controlling water chemistry The Alps are dominated by calcareous rocks and calcite dissolution contributes Ca2+ to the water.Brabant in the Netherlands has sandy soils (quartz) that are nutrient poor. Rainfall is slightly acidic and contributes some SO42-.The rift valley in Kenya has lakes fed by run off from the surrounding highlands before the water is then subject to strong evaporation in this arid climate. This causes minerals to become supersaturated and precipitate from the water e.g. calcite precipitation may explain the low Ca2+ concentration...
• groundwater recharge chemical data can also be used to calculate the annual rate that a groundwater reservoir is replenished. Again Cl- is used as a conservative ion. The data in the table above is for a site where the geology is a fluvioglacial sand composed of 95% quartz and a few other reactive minerals. As the local geology is sandstone (composed of quartz, SiO2) the contribution to the composition of the water from dissolution of minerals in the sandstone is anticpated to be very small. Chloride has been concentrated by 3.8 times compared to the rainfall composition. The average rainfal...
• what is water quality affected by?

  evapotranspiration

  transpiration

  selective uptake by vegetation

  redox (oxidation-reduction)

  cation exchange

  mineral dissolution

  secondary mineral precipitation

  water mixing

  leaching (e.g. of manures or fertilisers)

  pollution

  biological processes in lakes/rivers
• name some key water quality parameters:

  temperature

  electrical conductivity

  suspended soils

  pH

  dissolved oxygen

  hardness

  nutrient loading
• units used for reporting water chemistry mmol L-1 = mg L-1 / gram formula weight mmol L-1 = ppm (part per million) x (density/gram formula wt) mmol L-1 = meq L-1 / charge of ion Gram formula weight = weight in grams of 1 mol of atoms or molecules (1 mol = 6.02 x 1023, Avagadro’s constant)
• dissolved solids in water strongly influenced by geology but other factors may be significant; atmospheric inputshuman activities (e.g. agricultural activities, urban run-off, waste disposal) seawater has more dissolved solids in it than rainfall rainwater hits rock, reacts with rock, and can pull solids out
• natural and anthropogenic sources of dissolved solids groundwater age, climate, and geology can affect the conc of dissolved solids dissolved minerals affect chemical properties; hardnessacidityconductivity these then affect physical properties; colourodourcapacity to support life etc
• conductivity of water ability to conduct electricity; linked to conc's of dissolved mineral saltsunits (mS cm-1) natural lakes and rivers 10-10,000 mS cm-1 ^ levels above this indicate pollution fewer dissolved minerals = lower conductivityhigher conductivity = polluted waters
• TDS (mg l-1) related to conductance; TDS = conductance x (a factor) the factor is generally in the range 0.55 - 0.75 and is determined for a specific water body 0.67 is considered the standard when the actual conversion factor is unknown the factor increases with increasing SO42- concentration
• pH of waters measure of acidity pH = - Log 10 (H+) unpolluted natural waters pH 6-9remains nearly constant for a given water bodyfield measurement using an electrodetemperature dependentuse a temp compensating electrode
• what controls pH in water more H+ ions = more acidic more OH- = more alkaline pure water doesn't have a lot of free ions available to react, the pH generally stays quite stable
• pH of rain Rain is naturally acidic: Contains dissolved atmospheric CO2CO2 dissolves in water in the atmos to form H2CO3pH of c. 5.6 Increased acid rain SO2 and NOx in the atmosReact with H2O, O2, and other chemicals to become H2SO4 and HNO3pH below 4.5Sources – burning fossil fuels, manufacturing, electricity generation, vehicles
• buffering capacity of rivers/lakes rivers/lakes in granitic regions - low buffering capacity (unable to neutralise acidity). very susceptible to acid rain etc in limestone areas - greater buffer capacity. can neutralise extra acidity add the same amount of acid to lakes of equal sizes; they would adopt different pH values due to their different buffering capacities
• effect of photosynthesis on pH aquatic plants can use CO2 or HCO3-during the day CO2 may becomes exhausted so HCO3- is used insteadOH- ions secreted to replace HCO3- hence the water becomes more alkalineat night when photosynthesis ceases, the water body returns to normal plants make it less acidic through CO2 removal
• other processes affecting pH oxygenation - leads to a decrease in pH. changes with respiration and decomp.denitrification / sulphate reaction - increase in pHturbulence precipitation / sedimentation. acid rain, water running through different geologies
• hardness of water the sum of the ions that can precipitate as 'hard particles' from water sum of Mg2+ and Ca2+ (and Fe2+ in some waters) Expressed as meq l-1 or mg CaCO3 l-1 hard water has a high mineral content, causes limescale and scum
• temporary vs permanent hardness Temporary hardness - often happens when you boil water the Ca2+ and Mg2+ concentrations that are balanced by HCO3- (meq l-1) and can therefore precipitate as CaCO3 and MgCO₃ Permanent hardness Ca2+ and Mg2+ concentrations in excess of HCO3- concentration (meq l-1)higher conc of ca and mg than hco3, ie when all hco3 used up, still ca and mg left
• dissolved oxygen in water Free, non-compound oxygen present in water Oxygen is not very soluble in water cf. to other gases (e.g. CO2) Solubility depends upon; • temperature (increase leads to a decrease in dissolved O2) • pressure (decrease leads to decrease in dissolved O2) • concentration of dissolved minerals (increase leads to a decrease in dissolved O2)
• saturation level of water saturation level: when volume of water in each direction becomes equal, the water is saturated with O2 (regardless of temperature) dissolved oxygen is measured in the field with an electrode
• stratified water bodies in stratified water bodies (i.e. little mixing) water must diffuse through the water column --> sets up an O2 gradient. tends to happen in summer when top of water is warmed, cooler denser waters further down, and thermocline in the middle. turbulence - increases mixing and breaks down the O2 gradient O2 can become supersaturated e.g. where photosynthesis is high animals, plants and some chemical reactions consume O2 and lead to an O2 deficit
• suspensed solids siltclaysmall particles of organic and inorganic mattersoluble organic compoundsplanktonother micro-organisms typicall 10nm - 0.1 mm diameter measured as the amount of material retained on a 0.45 mm filter paper suspensed soils can lead to turbidity as they cause light to scatter
• sediment-water interface Boundary between bed sediment and the overlying water column. Physical (storms), biological (worms,plant roots), and chemical processes occur at the sediment-water interface. Exchange of material between sediments and water column is important for controlling the composition of lakes - nutrients can be pulled up, change composition of water Diagenetic reactions (reactions within sediment after burial) are biogenic and abiotic and influence water chemistry significantly
• biogenic reactions Decomposition of organic matter, results in: Removal of; dissolved O2, reduction of NO3-, SO42-, HCO3- Production of; CO2, NH4+, PO43-, HS- and CH4 these changes may then bring about further changes in the sediments e.g. solubilisation of Fe and Mn where O2 has been removed
• Remineralisation of organic N and P results in increasing NH4+ and PO43- below the sediment-water interface Increasing alkalinity as a result of decomposition of organic matter directly (organic N to NH4+) or indirectly (CaCO3 dissolution to release CO2 as organic C is re-mineralised)
• fluxes sedimentation of solid particlesentrapment of water in pore spacesupward flow of pore fluid due to hydrostatic pressure gradients (deep water body and pressure leads to pushing sediment up)molecular diffusionbioturbation (movement or burying of little organisms in the sediment) warmer = higher sedimentation rates
• nutrient cycling the movement and exchange of organic and inorganic material back into the production of living matter mostly recycled: inorganic forms taken up by primary producersconverted to organic formspassed up food chainreconverted to inorganic forms by decomposers
• vegetation - impact on water chemistry evaporation and transpiration increased conc of ions in proportion to the amount that evaporatesCl- can be used to track this as it behaves conservatively decay of OM in aquifiers can lead to reduction of Fe-oxides, sulphate and nitrate or methane formation. increases in CO2 may increase weatheringselective uptake of ions by vegetation and storage in biomass
• anthropogenic effects industry - acid rain etclandfill leachateagricultural activitieshigh nitrate levels from fertilisers and manures etc
• water sampling deterioration water samples can rapidly deteriorate during and after sampling leaching of contaminants from the surface of the containerleaching of organic compounds or metal ions from the containeradsorption onto container surfacereaction of the sample with the containerother sample changes e.g. degassing, particulate - solution exchange, decomposition such processes must be minimised if reliable results are to be obtained