GROUNDWATER QUALITY

Groundwater and Soil quality• Quality of GW as important as its quantity

• Usability of GW depends on the quality: chemical, physical, and bacteriological

• Need studies: source and environment of GW, pollution sources, dan other aspects related to the quality of GW

• Quality of GW is affected by the quality of soil that holds the GW

• Effect of Soils and Plants– Minerals mobilize in the soil mantle as clay

and other soil particles weather, and planta and animal materials decompose rains make them leach to GW

– Humid areas salt concentration ≈400 – 500 mg/l leachate from an agricultural field

– Evaporation of water from surface and take up water by roots of plants increase the salt concentration

– The dominant ions Ca, Mg, Na, HCO3, SO4, and Cl.

• Leachate from an irigation field in an arid climate several thousand milligrams per liter

• Soil and plant effects ion exchange and other reaction of the water and its constituents with the soil, and uptake the nutrients by root plants.

• Ion exchangecations which are adsorbed and exchanged for other cations by the negatively charged clay and organic matter

• The sequence: Li, Na, K, NH4, Rb, Cs, H, Mg, Ca, Sr, and Ba.– i.e.: Li is replaced by Na, Na by K, etc.

• Dense clay layers (aquitards and aquicludes) act as semi permeable membranes that hold back certain ions while passing others

• Chemical reactionsimmobilization of phosphate adsorbed and precipitated in most soils other than pure sands

• Heavy metals can be fixed in the soil, particularly when the soil contains clay, has a pH above 7 and is aerobic

• Alkaline soils: calcium CaCO3, acid rain water, however, can dissolve CaCO3 in the soil

• Plan rootscan absorb nutrients like N, P, K, S, and certain heavy metals and other trace elementsits concentration will be reduced by the time the rainwater has reached the GW.

• Chemical fertizersenrich the content of N, P, an K

• Canbon dioxide and organic acids produced by plants and other living materials in the soillower the pH of water accelerate the rate of weathering and enhances the mobility of metals

• Aquifer effects– Water continuously moves downwardreacts with

soil and rock materials in the vadoze zone and aquifer these reaction primarily consist of solution of solid phase in accordance with the solution chemistry of the particular minerals range from almost insoluble to very soluble (evaporites) that are affected bytemperature and pressure

– Other sources of dissolved saltsunderground waters of marine origin and salty connate waters intrude freshwater aquifers.

– Clay materials behave like membranes to screen water which can produce osmotic-pressure differences, salt seiving or ultrafiltration, and electric-poential difference

– Meteoric GW tends to be of good quality except where it has been stagnant or otherwise isolated from the hydrologic cycle in mineral rich or clay rich aquifers• E.g.:igneous and crystaline rocks generally yield

excellent GW quality (salt content <100 mg/l; seldom over 500 mg/l)

Bacteriological quality• Bacteria and micro-organism:

- Harmless- Diseas-causing pathogenic bacteria

• Natural water, except very shallow aquifers, free from pahogenic bacteria or viruses many consumers treated GW

• Natural water polluted by polluted water and may lack of chlorination or other disinfection

• Surface soils contain a myriad of microorganisms decrease rapidly below the root zone

• Microbiological activity can and will exist in many subsurface regions (e.g: extend to a depth of about 2000 m, assuming a normal temparature increases about 3oC per 100 m).

– An example: coliforms present in the interstines of human beeing and other warm-blooded animals. They are harmless however their presence in GW indicates the precense of pathogenoc bacteria

– BOD (biological oxygen demand) an indirect index of the presence of organic matter and waste

Chemical quality-Water is an excellent solvent

º knowledge of the geochemistry of the dissolved constituents

º knowledge of methods of reporting of analytical data

• Main constituents on groundwater:– TDS– Hardness– Alcalinity, Acidity, pH– Other constituents: Calcium, Magnesium,

Iron, Potassium, Strontium, Manganese, Aluminum, Phosphorous, Nitrogen, Chloride, etc

– Minor and trace mineral: small quantity but sometimes dangerous (toxic), e.g. Arsenic

Reporting chemical analysiso Concentration of dissolved salt or ions in

water expressed by (i) weight and weight-volume relative weights of

the solute and the solution in parts per million (ppm),i.e. One part by weight of the solute in one million part by weight of the solution ≈ miligrams in one liter (mg/l) for low concentration of dissolved solid; however, for high con it needs to be corrected

(ii) chemical equivalent combination and dissociation of cations and anions are governed by their equivalent weights (combining weights) rather than their gravimetric weight.

The equivalent weight of an ion equals to its atomic or molecular weight devided by its valancy. By deviding ionic concentration (ppm) by the equivalen weight of that ion resulting the concentration in equivalents per million (epm), or more precisely in milligram equivalents per kilogram

If the original data are reported in milligrams per litre resulting: milligram equivalents per litre (meq/l)

Since one equivalent weight of cation will combine with one equivalent weight of anion the sum of the cations should equal to the sum of anions when they are expressed in epm. Otherwise, errors or incompletness of chemical analysis are indicated

Analytical results are sometimes expressed in terms of hypothetical combinations by combining milliequivalents of the cations with the anions in the following sequence:

Cation Anion

Calcium Carbonate

Magnesium Bicarbonate

Sodium Sulphate

Potassium Chloride

Nitrate

Konsentrasi dalam satuan mg/l x faktor konversi pada tabel = meq/l

Sumber: Todd, 1980

DIAGRAM BATANG

DIAGRAM LINGKARAN

DIAGRAM POLA

DIAGRAM VEKTOR

DIAGRAM PIPER

• Dissolved constituents in GW– Silica from quartz (the cristalineof silica),

feldspars, mica, silicate minerals, etc a few – 60 ppm

– Iron from pyroxenes, amphiboles, etc in the form of ferric hydroxide, < 0.5 ppm

– Manganese from metamorphic and sedimentary rocks conc < 0.2 ppm

– Calcium– Magnesium– Etc...