The Islamic University of Gaza Faculty of Engineering ...site.iugaza.edu.ps/ymogheir/files/2010/02/Lecture-8-2010.pdf · by clay minerals, organic compounds by ions in solution, often

The Islamic University of GazaFaculty of Engineering

Civil Engineering DepartmentWater Resources Msc.

Groundwater Hydrology- ENGC 6301

lecture 8

Groundwater Quality &Groundwater Quality Modelling

)[email protected]. Yunes Mogheir (Instructor:

Semester: 1st/2010-2011

mailto:[email protected]

GROUNDWATER QUALITY

Part 1

GROUNDWATER QUALITY • Major objective in groundwater resources exploitation and management is to allocate groundwater in the right quantity and quality at the right time at the right locations.

• today's man-induced agricultural, industrial and domestic pollution most often the dominant factor in water supply.

•Modelling of groundwater quality simulates the behaviour of the considered constituents in time and space and thus provides a tool for future prediction of the effects following from various scenarios of pollution loads and changing groundwater regimes.

• Monitoring of groundwater quality gives an indication of its present suitability for domestic, agricultural, recreational, industrial and other uses

Groundwater pollution

• Sources of pollution are of natural or man induced origin

• Examples of natural sources are : - intrusion of salt sea water, - leaching from weathered surface rocks in the unsaturated zone,- dissolution from the soil and rock matrix (solid interface).

• Artificial pollution is predominantly of domestic biological and of industrial chemical origin. Another harmful contaminant is nuclear waste.

• Climatologically and land use conditions determine the natural recharge and temperature the rate of chemical weathering and degree of dissolution of the soluble constituents

•The unsaturated zone is probably the most reactive environment because of the presence of CO2 (carbonic acid) and oxygen.

Groundwater pollution

• In the saturated zone the concentration of a solute constituent is effected by different biological and chemical processes, adsorption, molecular diffusion and mechanical dispersion.

The most common reactions

• uptake of CO2 and O2 in the unsaturated zone • dissolution by CO2 of carbonate and, to a lesser extend, alumino silicates •cation exchange oxidation of organic matter,• reducing O2:, N03 and S04 • oxidation of FeS: with precipitation of Fe(OH)4• dissolution of evaporite salts like NaCI and Ca(SO)4

PROPERTIES OF GROUNDWATER

• Groundwater is a complex chemical solution in a dynamic state. The entire structure of its molecules makes water a good solvent. Its H+ ions weaken or even bind the OH- ions of other constituents.

•• The acidity (Carbonic acid ) may result from :- the solution of CO2 in infiltrating water. - CO2 may originate from the atmosphere and Rain water - but will come in larger quantities from living organisms or oxidation of organic maners.


• Nitrification processes are induced by fertilizers and animal manure which contain ammonium NH3, which is oxidized to NH4 and further to nitrite HNO2 and nitrate HNO3 (nitrification).

• Ion exchange involves the replacement of ions adsorbed by clay minerals, organic compounds by ions in solution, often called base exchange or cation exchange

•Hard calcium bi-carbonate water may reduce to soft alkaline sodium rich water when flowing over a clay formation, as calcium (Ca) and magnesium (Mg) is replaced by sodium (Na).


• The composition of groundwater changes with depth. At larger depths the infiltration effect and the groundwater transport velocity is lower or even stagnant, resulting in a reduced dispersion or mixing process.

•Deep groundwater (fossil water) is less suitable for municipal water abstraction due to the high concentrations of chlorine, fluor or other constituents, applies for many aquifers

• Pesticides used in agriculture percolate by the excess irrigation water to the groundwater and may be partly adsorbed by clay particles. By adsorption the chemicals are accumulation in the soil and sooner or later will be leached to the groundwater.


• Pollution sources of more recent origin - like municipal land fills and hazardous industrial waste dumps -in general have not yet percolated deeper than the top part of the aquifer.

• Another potential hazardous pollution is the long term use of fertilizer containing very small quantities of heavy metals - like cadmium - in soils containing even small quantities of organic clay and with a shallow water table.

•The high concentrated cadmium dissolves and will percolate to the groundwater

GROUNDWATER QUALITY DETERMINANTS

• The temperature in groundwater fluctuates according to the temperature of the infiltrated water to a depth of 10 m in the tropics to 20 m in polar regions. At greater depths the temperature increases by 1°C per 30 m due to the geothermal gradient.

• Colour in general is caused by organic and mineral matter in solution.

•Turbidity is mainly caused by suspended and colloidal material like clay and silt particles, but also ferric hydroxides.

• Taste and odours are caused by mineral and organic matter, bacteria or dissolved gases.

•Hazardous pathogenic bacteria, viruses and micro organisms are in general not native to the subsurface environment

1. Physical and biological determinants :


• Chemical elements in groundwater originate from: the infiltrated and injected water, biochemical processes in the saturated zone andbreakdown of minerals in the rock.

- The threat of a constituent relates to its hazardous characteristics and its concentration. Even small concentration may be harmful in water supply.

- Natural chemical constituents in rain water are CI (0.1 to 100 mg/I), Na (0.1 to 50 mg/l), Ca and S04 (0.1 to 20 mg/l) and Mg (0.1 to 10 mg/I).

-Man induced chemical constituents as a result from biological nitrification and use of fertilizers, and carbon dioxide are converted to week acids which attack the parent rock material.

2. Major natural chemical determinants :


Rock type TDS Ca Mg HC03 Na K CI S04 Fe Si

04

Granite

223 27 6.2 93 9.5 1.4 5.2 32 1.6 39

Sandstone 210 40 12 67 7.6 .4 19 26 12

Limestone 247 48 5.8 168 4 .7 .1 4.8 .05 8.9

Groundwater constituent concentrations (mg/l) at different rock types

GROUNDWATER QUALITY DETERMINANTS Minor chemical constituents, tracers and radionuclides.

• Minor constituents in principal do not to exceed 1mg/1. Various Some of them are a matter of concern because of their toxic effects. The most well know are arsenic, barium, copper, chromium, cadmium and mercury. Aluminium is highly insoluble with concentrations less than 0.5 mg/1.

less than 0.2 rng/l, originating from weathering 0.1 to 10 mg/l, dissoluted from solid interface 1 mgll per 300 mg/l of chloride. from decay of plants and animals 0.1 to 10 mg/I, from volcanic rocks 0.01 to 10 mg/l, limited by calcium rich clays

Manganese: Fluoride:Bromide:Iodine:Boron:Strontium:

GROUNDWATER QUALITY DETERMINANTS Acceptable constituent concentrations for municipal and livestock use

Physical Colour (units) 15 Filtrable 500 mgll Turbidity(units) 5 Hardness 2-50 meqll

Biologic Coliform bacteria Nil Virus 1 plaque/lOl Micro organisms Nil

Chemical Bicarbonate (HC03) major Calcium (Ca) 75-200 (mgll) Chloride (CI) 200-600

Magnesium (Mg) 30-125 Hazardous Nitrate (as N03) 50-100 Sodium (Na) 200 Sulphate (S04) 250 Silica 50

Part 2:

GROUNDWATER QUALITY MODELLING,

ANALYTICAL SOLUTIONS

• Aquifer clean-up means the decrease of the concentration of a pollutant in an aquifer to acceptable levels

AQUIFER CLEAN-UP, SINGLE CELL APPROACH

GROUNDWATER QUALITY MODELLING,

GROUNDWATER QUALITY DETERMINANTS AQUIFER CLEAN-UP, SINGLE CELL APPROACH (Convective flow and Advective Transport)

LONGITUDINAL AND TRANSVERSAL DISPERSION

• Dispersion is the spreading of a solute contaminant from the average flow path.

•Dispersion in the direction of the flow is called longitudinal, and perpendicular to the resulting or average flow direction is called transversal dispersion .

Taking the dominant or average flow direction along the x-axis, then the longitudinal dispersion can be described as

LONGITUDINAL AND TRANSVERSAL DISPERSION

• Accumulation of a substance (adsorbate) on a solid interface (rock, soil) is induced by electrical attraction, intermolecular forces, chemical reaction, and the like.

• In the saturated phase the constituent is partly in the fluid phase (water) and partly on the solid surface.

ADSORPTION TO THE SOLID INTERFACE

• The adsorption isotherm (F) relates the quantity (mass) of the adsorbed species on the solid interface to the mass of the same species in the fluid phase, at a defined temperature.

•This isotherm can represent both the equilibrium state or a non-equilibrium state, and be linear or non-linear


• We should study the mass balance as follows:



Decay Processes(Radioactive decay)

Chemical Transport

Decay Processes(Radioactive decay)

-Assuming advection only is in many cases a reasonable approach. -Dispersion can be added in a later stage of a study. -Diffusion is only significant under low flow conditions.

Part 3:

Groundwater Pollution Transport

Numerical Modelling


• The mechanisms affecting the transport of a pollutant are advective, dispersive and diffuse fluxes, solid-solute interactions and various chemical reactions. These processes take place simultaneously

The general groundwater-transport process can be considered to be composed of •transport due to flow as described by Darcy and storage • dispersion due to molecular forces and local changes in velocity.• interactions by (bio )chemical processes - external flows, sources and sinks - adsorption to the solid interface


• In general convective fluid flow is described by the Darcy equation and is the dominant part of the mass balance.

•Transport of solutes by this convective flow is called advective transport of solute contaminants.

•The spatial 'natural' spreading of a pollution is called hydro-dynamic dispersion and is caused by mechanical dispersion due to local differences in flow distribution (irregular pore and fracture structure) and by molecular diffusion being the physical tendency to flatten out potentials like heads and concentration.

GENERAL EQUATIONS AND NUMERICAL APPROACH

• Three major mechanisms can be distinguished which effect the spatial and temporal distribution in concentration:

1. Transport according to Darcy, yielding the advective transport component

2. Hydro-dynamic dispersion: molecular diffusion and transport dispersion

3. Interactions: - External pollutant inflow and outflow –- Internal reactions, both chemical and

biological

Documents

The Islamic University of Gaza Faculty of Engineering ...site.iugaza.edu.ps/ymogheir/files/2010/02/Lecture-8-2010.pdf · by clay minerals, organic compounds by ions in solution, often