presentation 4 hydrogeologie.pdf

Hydrogeology HYDROGEOLOGY 4 E SEMESTER M AY 20 2 4 Hydrogeology Onderwerpen die aan de orde komen:  Hydro(geo)logical cycle  Rocks and groundwater  Water balances  Groundwater Chemistry  Grondwater management  Grondwater in Suriname Groundwater Quality Groundwater Chemistry Th...

Hydrogeology HYDROGEOLOGY 4 E SEMESTER M AY 20 2 4 Hydrogeology Onderwerpen die aan de orde komen:  Hydro(geo)logical cycle  Rocks and groundwater  Water balances  Groundwater Chemistry  Grondwater management  Grondwater in Suriname Groundwater Quality Groundwater Chemistry The groundwater chemistry can be evaluated by considering flow systems ( recharge and discharge areas). Recharge Unsaturated Saturated Discharge from zone rock area precipitation Groundwater Quality Groundwater Chemistry Chemical composition of rainwater Precipitation contains natural and industrial components: -Natural components; water in oceans and seas, transported through the atmosphere to the continents. ( the chemical composition is similar to the composition of rainwater). The dominating natural components in precipitation are usually Na and Cl. These amount in precipitation decrease with the distant from the cost. Groundwater Quality Groundwater Chemistry Chemical composition of rainwater - Industrial Components: these components are influenced by heavy and chemical industries and mining operations. The dominating components are usually Sulphur (S) and Nitrogen (N). Groundwater Quality Groundwater Chemistry Processes in the unsaturated zone As precipitation infiltrates it’s way through the unsaturated zone, the following processes are playing a role: Increase in concentration by evaporation; Decay of organic matter; Uptake of chemicals by plant roots; Dissolution of minerals (carbonate dissolution and silicate weathering)  Groundwater Quality Groundwater quality is the state of the water beneath earths surface. The usefulness of groundwater is usually determined based on the following characteristics: -Chemical : - Inorganic constituents (salt, nutrients, trace elements) - Organic constituent (Pesticide, Benzene, organic carbon) -Physical: - Temperature, turbidity, color, taste, odor -Biological (Bacterial) - Pathogen indicator organisms ( Coliform, E.Coli) - Pathogens ( Bacteria, Virus) Groundwater Quality Chemical composition of groundwater: -Naturally -Manmade -Large quantities -Trace concentrations  Groundwater Quality Groundwater Chemistry Processes in the Groundwater system: -Dissolution of minerals along a flowpath -Redox processes along a flowpath -Ion exchange in the groundwater system -Flushing of saline groundwater  Groundwater Quality Some main chemicals found in groundwater: PH Measured to give an understanding of the acidity or alkalinity of groundwater. The ph of groundwater is important because it controls most chemicals reactions which take place. Electrical Conductivity (EC) EC in groundwater is measured to indicate the amount of dissolved solids or salts in groundwater. The more dissolved salts available in the groundwater, the higher the measured EC. Dissolved Oxygen Amount of oxygen present in groundwater depends on the aquifer source. Confined aquifers contains no oxygen, while unconfined aquifers can be fully saturated with oxygen. Alkalinity The alkalinity level of a groundwater describes its ability to neutralize present acids. Alkalinity level in groundwater is caused by carbonates and bicarbonates. Groundwater Quality Hardness Hardness of water refers to the amount/ concentration of Magnesium and Calcium present in that water. Salinity Groundwater Quality Groundwater quality is affected by: -Seepage from soil/streambed -Rock or sediment through which the groundwater flows Based on the quality of the groundwater, it can be suitable for: -Drinking water purposes -Domestic purposes -Irrigation purposes Groundwater Quality Source:marlborough.govt Groundwater Quality Why is groundwater quality important: -Health risks -Ecological impacts -Industrial uses -Aesthetics -Agricultural uses -Drinking water supply -Impact on aquifer properties Groundwater Quality Groundwater contamination Contamination is the introduction of any solute in undesirable concentration in groundwater rendering it unfit for its intended purpose of use. Groundwater Quality Groundwater contamination Most common types of groundwater contamination are: Environmental (natural) Anthropogenic ( influenced by human origin) ◦ Domestic ◦ Industrial ◦ Agricultural Groundwater Quality Groundwater Contamination Environmental contamination: -Seawater intrusion -Arsenic originating from rock minerals -Fossil brackish groundwater -Fluoride and heavy metals released from rock minerals  Groundwater Quality Groundwater Contamination Domestic Contamination: -Sewerage; liquid waste from sewerage pipes and septic tanks leaking into aquifers. - Landfills; Leakage of contaminants mobilized by percolating rainwater. Industrial Contamination: - Waste; Liquid and solid waste leaking into the aquifer. Agricultural contamination: - Agro- chemicals; fertilizers and pesticides washed down by rain into the aquifers. Groundwater Quality Groundwater Quality Groundwater Quality Groundwater in areas where population density is high and human use of land is intensive are more vulnerable to contamination. Effects of groundwater contamination: Poor drinking water quality Loss of water supply Degraded surface water systems High cleanup cost High cost of alternative water supply Potential health problems Groundwater Quality Chemical processes in groundwater - Dissolution - Redox reactions Groundwater Quality Dissolution processes: Dissolution is the formation of a solute from a mineral contained in solid rock. aA + bB(s) cC + dD a,b,c,d = numbers to equalize total atoms on left and right hand side of the equation A,C,D = ions in solution (as solutes) B(s) = solid mineral Groundwater Quality These reversable processes usually end up in a state of equilibrium: “The amount of mineral dissolved equalizes the amount that precipitates” The following equilibrium constant has been defined: Kmineral = Kmineral = equilibrium constant [C], [D], [A] = concentration of ions in solution (moles/m3) = solubility product of ions in solution. Groundwater Quality Example : Dissolution and precipitation of the solid mineral gypsum CaSO4 Ca²⁺ + SO4²⁻ KCaSO4 = [Ca²⁺ ] [SO4²⁻]= 10⁻⁴⁶ Solution is sub-saturated: If concentrations of ions in the groundwater are below concentrations at equilibrium Solution is in a state of super-saturation: when the concentration of ions are above the concentrations at equilibrium. Groundwater Quality Other dissolution processes involves: - Carbonates and carbon dioxides - dissolution of silicate minerals Groundwater Quality Redox reactions Redox processes which can be presented by redox reactions play a major role in the chemistry of natural groundwater systems. Redox reaction = reduction – oxidation reaction In these reactions there is a transfer of electrons from one atom to another atom. Reduction = the valency of one atom is reduced Oxidation = valency of one atom is increased Groundwater Quality Groundwater Chemistry Groundwater chemistry can be evaluated by considering groundwater flow systems ( recharge and discharge areas). Groundwater chemistry depends on the flow path which is followed by a given water particle: -Recharge from precipitation -- unsaturated zone -Saturated rock -Groundwater discharge area Groundwater Quality Evaluation of groundwater chemistry depends on: - Changing geological environment over time periods - Contaminants entering the groundwater system Groundwater Quality Chemical composition of rainwater It is important to understand the chemical composition of precipitation water in order to understand the effects on groundwater chemistry. Precipitation contains: - Natural components - Industrial components Groundwater Quality Natural components: Originates from water in he oceans and sea which are transported to the continents ( usually the composition of rainwater is similar to the composition of sea water, but in much lower concentrations). Dominant components in rainwater: - Sodium (Na) - Chlorine (Cl) These components are usually present in dissolved form. The amount of Na and Cl decreases with the distance from the coast. Groundwater Quality Industrial components Originates from heavy and chemical industries and mining operations. Dominant (substantial) components: - Sulphur (S) - Nitrogen (N) These substances turn the precipitation aced and influence the dissolution process in the groundwater system. Groundwater Quality Groundwater chemistry in the groundwater system Groundwater Quality Typical processes in the unsaturated zone The chemical composition of the recharge water in the unsaturated zone is influenced by the following processes: -Increase in concentration by evapotranspiration : direct evaporation and transpiration of plants affects the concentrations of ions dissolved in recharge water. - Decay of organic matter: remains of plants and other forms of decaying organic matter influencing the composition of the recharge water. This process is usually an redox reaction, whereas the organic matter is oxidized and carbon dioxide (CO2) is being released. This results in ana increase of CO2 in the recharge water. Groundwater Quality Typical processes in the unsaturated zone - Uptake of chemicals by Plant roots: depending on the crop/plant variety they suck up water from the unsaturated zone and selective chemicals. This may lead to removal of chemicals from the recharge water and leading to a change in chemical composition. - Dissolution of minerals: Rock material in the unsaturated zone can be dissolved by recharge water. Principal reaction occurring in these situations are carbonate dissolution and silicate weathering in an acid environment. If no other process are taken place simultaneously, the dissolution increases the rock-related chemicals in the recharge water. Groundwater Quality Processes in the saturated groundwater systems - Dissolution of minerals along a flow path: the concentration of ions dissolved in water usually increases along a flowpath. -Redox processes along a flowpath: the redox potential usually decreases along a flowpath in the saturated groundwater system. Oxid- conditions change into anoxic conditions. -Ion exchange in the groundwater system: groundwater traveling along a flowpath in a flowsystem may permeate through rock layers consisting of clays, iron oxides and organic material. These materials have high ion exchange capacity. Presence of these layers may substantially chane the groundwater chemistry. Groundwater Quality - flushing of saline groundwater: this is a typical process that relates to the geological history of a groundwater system. The flushing of saline water transforms the groundwater systems into a less saline system, and brackish or freshwater environments are established. Groundwater Quality Processes at groundwater discharge points: At locations where groundwater discharges, one may observe the precipitation of dissolved minerals. - Precipitation of minerals in groundwater discharge areas may also occur as a result of an increase in redox levels triggered off by the introduction of oxygen. - precipitation of minerals can also occurs due to high evapotranspiration rates at locations with high groundwater tables.