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2. Fundamentals of Groundwater and Hydrogeology

INTRODUCTION

Hydrogeology is the science of the occurrence, distribution, and movement of water

below the ear th 's s u rface. Water is present b elow the ear th’s su rface in na tu ral ly

occur r ing spaces (intergran u lar voids, spa ces, an d oth er fractu res) within soil and rock

forma tions . Th e water th at exists a n d moves below th e eart h’s su rface is called

groundwater. A common m isconcept ion is tha t groun dwater occurs in u nd ergrou nd

rivers; however, as explained in the following sections, groundwater occurs and moves

qu ite differen tly th an su rface wat er (e.g., rivers). Groun dwat er m oves relatively slowly;

a foot a day would be a high velocity; whereas surface water flow rates can be on the

order of a foot a secon d for a ra pidly flowin g stream . Su rface water typically spen ds

less than a few weeks in a given river or stream; the average residence time of

groun dwat er is estimated t o be 280 year s (Lvovitz, 1970 ). The ra mification of th isdifference is th at once a groun dwater s ource becomes polluted, i t may be centu r ies

before th e system will flush itself of th e conta min ation (Cleary, 1 990 ).

Str ict ly defined, grou n dwater is su bsu rface water th at exis ts in the z on e of s a tu ra tion ,

which is th e zon e in a soil colu mn where a ll open ings are full of water (Figure 2 .1). Th e

zone of sat ur at ion is d is t ingu ish ed from th e unsaturated (vadose) zone . The voids in th e

un sa tu ra ted zone conta in both water and a i r . The un sa tu ra ted zone begins a t the

ear th ’s su rface an d extends ver t ical ly down to the sa tu rated zone. The water ta ble is

the top of the satu rated zone; in some cas es, it actu al ly r ises a bove the groun d su rface,

as in wet lan ds.

About 94 % of the ear th 's water is saline an d foun d in th e oceans a nd s eas. Althou gh

groundwater accounts for only 4% of the total water balance of the earth (see Table 2.1),

i t const itutes a bou t two-th irds of th e fresh water resou rces. If we ignore th e fresh water

t ied u p in ice caps an d glaciers , ground water accoun ts for almost a ll of the ea r th 's

fresh water (n ons aline) availab le for u se.

THE HYDROLOGIC CYCLE

The term hydrologic cycle refers to the complex interaction of surface, atmospheric, and

su bsu rface water (Figure 2.2). The interdepen dence of these water regimes is importan t

in u nd erstan ding the or igin of groun dwater . Groun dwater is foun d in water-bear ing

rock form ations , called aquifers , below th e eart h ’s su rface. Thes e forma tion s act not

only as r eservoirs for s tor ing water , bu t also as condu its for t ran sport ing water . In th e

hydrologic cycle, practically all water enters subsurface rock formations through the

grou n d su rface. This process is cal led grou ndwa ter recharge . The principal sou rces of

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water for recharge are precipi tat ion an d su rface water bodies su ch a s s t r eams , lakes,

an d r eservoirs .

After en tering su bs u rface forma tions , groun dwat er tra vels s lowly for varying dista nces

u nt i l it re turn s or discharges to the ear th ’s su rface via na tu ral or artificial mean s.

Man y st reams are formed an d flow primari ly from groun dwater discharge. Natu ral

discharge of groun dwater occurs m ost ly as flow into su rface water bodies . Natu ral

discharge direct ly to the groun d su rface is cal led a s pr ing. Water m oves u pward b y

evaporat ion from su rface water bodies an d t ra ns pirat ion of moisture f rom plants to the

a tmosphere .

Some groun dwater ma y also move up ward from th e satu rated zone into th e vadose zone

where it evaporates or tran sp ires. Artificial discha rge of groun dwat er occu rs m ainly

throu gh pu mp ing from wells .

Table 2.1. Estimate of the Water Balance of the World

Surface area Volume Volume Equivalent Residence

Parameter (km2) x 106 (km3) x 106 (9%) depth (m)* time

Oceans and seas 361 1370 94 2500 ~4000 yrs

Lakes and

reservoirs 1.55 0.13 <0.01 0.25 ~10 yrs

Swamps <0.1 <0.01 <0.01 0.007 1-10 yrs

River channels <0.1 <0.01 <0.01 0.003 ~2 wks

Soil moisture 130 0.07 <0.01 0.13 2 wks - 1 yr

Groundwater 130 60 4 120 2 wks - 10,000

yrs

Icecaps/glaciers 17.8 30 2 60 10 - 1000 yrs

Atmospheric water 504 0.01 <0.01 0.025 ~10 days

Biospheric water <0.1 <0.01 <0.01 0.001 ~ 1 wk

Source: Nace, 1971

* Computed as though storage were uniformly distributed over the entire surface of the earth.

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AQUIFERS AND NONAQUIFERS

Aquifers — Although groundwater occurs in many types of geologic formations, aquifers

are th e most significan t reservoirs of groun dwater . An a qu ifer is a wat er-bear ing layer

of rock or soil th at will yield water in a u sa ble qua nt ity to a well or spr ing. This imp lies

th at aqu ifers h ave th e abili ty to store an d to tra n sm it water. Aqu ifers are genera lly

clas sified in to two differen t types: confined a n d u n confin ed (Figu re 2.3). An unconfined

aquifer is one in which the upper surface of the water (called the water table) is at

atm ospheric pressu re an d, th erefore, is f ree to r ise an d fall with chan ges in a tmosp heric

pressu re an d th e volum e of water in s torage within th e aquifer . A confined aquifer

exists where groun dwater is confined u nd er pressu re greater th an a tmosph eric

pres su re. This is accomp lished b y th e presen ce of a relatively imperm eab le rock or soil

layer (confining lay er ) overlying the aqu ifer. When a well pen etrat es a con fined aq u ifer,

the water level in the well rises until the pressure is equal to the hydrostatic or

lithosta t ic pressu re imp osed u pon i t . If the water in th e well r ises above the groun d

su rface, a flowin g (art esian ) well resu lts.

An aquifer with essentially the same hy drau lic cond uctivity value (ab ili ty to tran sm it

water) thr oughout a cer tain area is said to be homogeneous . If h ydrau lic condu ctivity

differs within th e sa me a rea in an aqu ifer , the aqu ifer is sa id to be heterogeneous . The

relative h ydrau lic cond u ctivities for various types of geologic m ater ials a re sh own in

Figure 2.4.

Confining Bed s — A confining bed is a layer of rock or soil that is relatively

impermeable; i .e., i t restricts the movement of groundwater either into or out of

u nd erlying or overlying aqu ifers. There are th ree general types of confining bed s:

aqu iclu de, aqu ifu ge, an d aqui tard. An aquiclude is a relatively imperm eable layer tha t

is satu rat ed with wa ter, bu t does not yield ap preciable qu an tit ies of water to wells. A

clay layer may serve as a n aq u iclud e. An aquifuge is a relatively imperm eab le forma tion

tha t nei ther contains n or t rans mits water . Solid grani te (with n o fractu res) is an

exam ple of an a qu ifu ge. An aquitard i s a satu rated bu t rock layer with low permea bi lity

th at slows grou n dwater m ovemen t an d does n ot yield water freely to wells. An a qu itard

may, however, transmit appreciable quantit ies of water to or from adjacent aquifers,

an d ma y serve as a groun dwater s torage zone. San dy clay may be an a qui tard.

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ROCK AND SOIL PROPERTIES

Two properties of rock and soil influence the distribution and movement of

groundwater : porosity a n d perm eability . Thes e properties in fluen ce the rock’s a bili ty to

s tore and/ or transm it groundwater .

Poros ity of a rock or soil is a meas u re of th e spaces in th e rock. Qu an titatively, th e

porosity of a rock or soil is measured in volumetric terms and is equal to the ratio of the

volu me of pore spa ces in th e rock to th e total volu me of th e rock. Porosity is u su ally

expressed as a percenta ge. For example, a rock containing 30% pore spa ces has a

porosity of 30%. Porosities can ra nge from n ear zero to over 50%, depen ding on th e

rock or soil type, arrangement and sorting of particles, degree of cementation, solution,

compa ction a n d fractu ring, an d other geologic factors . With in a specific rock or soil

type, porosities can also vary widely due to variations in grain size, cementation, and

fracturing.

Permeability is the ca pa city of a rock, s edimen t, or soil to tran sm it fluids (e.g., water or

oil). More sp ecifically, perm eability is th e mea su rem ent of fluid flow th rou gh th e soil

medium . For a rock to be permeable, it s pore spa ces mu st be interconn ected. By

definition, a perm eable rock is porous . However, all porou s rocks are not necess arily

permea ble. If a rock conta in s ma ny pore spaces bu t none of them a re interconn ected,

then the rock is porous bu t not perm eable. Table 2.2 shows typical soil an d rock types

and their general ranges of permeability and porosity.

Permeability is an intrinsic property only of the rock or soil material and is independent

of fluid properties. Hy drau lic cond uctivity refers to the water-transmitting property of a

rock or soil ma terial . Figure 2 .4 pr ovides hydr au lic cond u ctivities for various geological

clas sifications an d Table 2.3 des cribes th e generalized hydrogeologic cha racter istics of

some m ajor rock an d s oil types.

GROUNDWATER FLOW

The m ovement an d factors control ling the m ovement of grou nd water are a n essen t ial

par t of understanding the nature and occurrence of groundwater .

Darcy’s Law

The n atu ral movement of groun dwater occur s in accordan ce with es tabl ish ed hydrau lic

principles. Hy d raulics is a bran ch of science th at d eals with the pract ical app licat ion of

l iqu ids in m otion. Th e factors con trolling grou n dwater flow were first express ed in 185 6

by the French en gineer Henry Darcy. The

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equat ion, kn own as Darcy’s law, can be expressed in i ts s implest form a s:

v = -K dh / d l

where,

v = t h e D a rc y ve loc it y or s p ec ific d is c h a rge (flow ve loc it y)

K = t h e h yd ra u lic con d u ct ivit y (a con s ta n t t h at s er ves a s a m ea s u re

of th e perm eab ility of a p orous ma terial)

d h/ d l

Groundwater flows from a higher hy draulic head to lower h ead or "downgra dient." Th e

groundwater elevation, typically measured in an aquifer using a well or piezometer,

repres ent s total h ydrau lic h ead. Total hea d (from Bernou lli 's equa tion ) for grou n dwat er

is actu al ly the su m of the elevat ion an d press u re head s. Velocity head in grou nd waterflow regimes is as su med to be n egligible du e to th e slow, lam inar flow.

Table 2.2. Water-Bearing Properties of Common Rocks

Permeability Porosity

Highest permeability Highest porosity

well-sorted gravel soft clay

porous basalt silt

cavernous limestone till

well-sorted sand well-sorted sand

poorly sorted sand & gravel poorly sorted sand & gravel

sandstone gravel

fractured crystalline rock sandstone

silt and till

clay cavernous limestone

dense crystalline rock fractured crystalline rock

dense crystalline rock

Lowest permeability Lowest porosity

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Table 2.3. Generalized Hydrogeologic Properties of Major

Rock and Soil Types

Alluvial Deposits Unconsolidated rocks and sediments deposited by streams or running water; size varies(boulders, cobbles, gravel, sand, silt, and clay. Most developed aquifers are alluvialdeposits (gravel and sand); highly permeable, often prolific aquifers.

Sandstone and Conglomerate Sandstone is sand deposit cemented and compacted into a consolidated rock;conglomerate is a consolidated gravel deposit; generally are not highly porous; therefore,usually not highly productive aquifers; generally not very permeable; when fractured,fractures can transmit water more readily, making rock unit more productive aquifer.

Clay Generally porous, but small pore spaces create high surface tension and combine withhigh tortuosity of the flow paths, to yield a very low permeability; clay beds generallyconfining beds, not aquifers, but restrict groundwater movement into or out of adjacentaquifers; shallow clayey soils can yield small quantities of water in domestic wells .

Limestone Porosity and permeability vary widely depending on nature of original rock and thedegree of alteration after deposition; limestone units that are important aquifers generallyare highly permeable due to dissolution of rock after deposition. Openings in limestonecan range from microscopic pore spaces to large solution caverns. Easily dissolved bygroundwater overtime because of calcium carbonate content; porosity and permeability of limestoneaquifers increase with time.

Volca nic Igneo us Roc ks Formed by eruption of molten lava; these rocks, particularly basalt flows, can form highlypermeable aquifers due to openings in the rock, including spaces and cavities betweenadjacent lava beds, shrinkage cracks, lava tubes, gas vesicles, fissures from faulting andcracking after rocks have cooled, and holes left by trees burned by lava.

Intrusive Igneo us and M etam orphic Roc ks Rocks (e.g., granite) formed below the earth’s surface by cooling of molten material; donot contain the openings found in volcanic igneous rocks; are practically impermeable.Metamorphic rocks consist of any type of pre-existing rock altered by changes intemperature and/or pressure, generally deep within the earth’s crust; relativelyimpermeable. Where intrusive igneous or metamorphic rocks are highly fractured orhighly weathered near the earth’s surface, yield appreciable quantities of water, butgenerally not prolific aquifers.

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Th is simp lified version of Darcy’s law is an idealized equa tion th at d oes n ot tak e into

account natural variations in porosity and permeability of a single geologic medium,

which produ ce irregu larit ies in flow. These variations can be accou nt ed for by u sing

other , more comp lex formu las. However, Darcy’s law ma y be u sed ra th er easily to

est ima te groun dwater f low rates .

Groundwater Flow Rate

Darcy’s law demon stra tes th at th e rate of groun dwater m ovemen t is controlled by th e

hydrau lic condu ct ivity of an aqu ifer and the h ydrau lic gradient . Natura l grou nd water

movemen t is qu ite slow. For examp le, a produ ctive allu vial aqu ifer with a hydr au lic

condu ct ivity of 100 f t / day an d a h ydrau lic gradient of 10 ft / 1000 ft h as a grou nd water

flow rat e of 1 ft / da y. Th is is ap proximat ely equ ivalent to 0.05 5 cm/ sec. Local

h ydrogeologic cond itions govern grou n dwat er velocities, an d values ran ging from

1 ft / year to 1 f t / day are considered norma l (Todd, 1980 ).

GROUNDWATER QUALITY

Understa nd ing the na tur e and occu rrence of groun dwater was firs t importan t to find ,

develop, and ma inta in rel iable water su pply systems. More recent ly, emph as is h as

sh ifted from qu an t ity to the qu al ity of the groun dwater resou rce, which var ies n atu ral ly

or can be n egat ively impa cted by h um an act ivit ies .

Natural Groundwater Quality

Groundwater quality reflects the nature of the aquifer in which it is found.Groundwater is affected by interaction with the soil or rock aquifer matrix, air, recharge

water, an d plant an d other organ ic activity. The natu ral quality varies widely bas ed on

the en vironm ental condi t ions. In s ome cases, th e "na tu ral" qua lity of groun dwater is

u na cceptable for dr inking withou t pretreatm ent . Water qua lity can be u na cceptable for

aes th etic (e.g., tas te or odor) an d/ or hea lth reas ons (e.g., n atu rally h igh ra dioactivity).

Common factors th at a ffect natu ral groun dwater qu al ity are discus sed below.

Dissolved Solids — Water has been called “the universal solvent” because it can dissolve

sm all am oun ts of almost al l su bsta n ces with wh ich i t comes in contact . Becaus e of this

abi lity, groun dwater contains varying am oun ts of dissolved sol ids . The qu an t ity and

types of dissolved s olids foun d in n atu ral , un polluted groun dwater depen d on:

• Chemica l compos it ion of ra inwater inf ilt r a t ing to the groundwater zone .

• Biologic and chem ica l reac t ions occur r ing on th e land su r face and in the so il zone .

• Minera l compos i tion of a l l o f the rock uni t s with which the groun dwater comes in to

contact .

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Different qualit ies of groundwater are required for different uses; the standards for

drink ing water , ind u str ial water, an d irrigation water all vary widely. For exam ple,

water u sed for s ome indu str ial appl icat ions mu st be of higher qua lity (greater pu ri ty)

tha n potab le dr in king water .

Th e concen tra tion of total dissolved solids (TDS) in groun dwater is an importa n t

ind icator of grou n dwater qu al ity. Water containing less th an 500 mg/ L TDS is

preferred for domest ic us e an d for man y in du str ial processes . Water containing more

tha n 2000 to 3000 mg/ L TDS is general ly too sal ty to dr ink; sea water cont ains

ap proximately 35,0 00 m g/ L. For exam ple, in the U.S., th e U.S. Geological Survey

(USGS) classifies water based on TDS as follows:

Table 2.4. Water Classification

Category TDS (mg/L)

Fresh < 1,000

Slightly saline 1,000 to 3,000

Moderately saline 3,000 to 10,000

Very Saline 10,000 to 35,000

Briny > 35,000

Sa linity — Sodium an d chloride compr ise sa lt (NaCl). In coas tal areas , saltwater from

the ocean can en ter aquifers an d add s al t to the groun dwater , a ph enomen on cal led

sal twater int ru sion. In inland areas , sodiu m an d chlor ide ma y be present in

grou nd water becau se th ey were t rapped in th e sedimen ts a t th e t ime of deposit ion.

Excessive sodiu m an d chloride give water a sa lty tas te. Large am oun ts of chloride

increas e th e corrosivenes s of water. Large concen tra tions of sodiu m can a dversely

affect people with cardiac problems , h igh blood pr essu re, an d cer tain oth er m edical

problems , and ma y dama ge some i r r igated crops.

Hard nes s — Two elements , calcium an d m agnesium , are m ainly responsible for the

ha rdn ess of water , which cau ses s cal ing an d deposi ts in water h eaters , boilers , teapots ,

an d other hot-water conta iners . The rock types that contr ibu te most to the am oun t of

calcium a nd m agnesium in groun dwater are limest one, dolomite, and gypsu m.

Limestone and dolomite also are responsible for high concentrations of carbonate (CO3)

an d bicarbona te (HCO3) in groun dwater . Carbona te an d bicarbonate control water’s

capaci ty to neut ral ize s t rong acids an d, together with calciu m a n d ma gnesium , caus e

water to be har d.

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Other — Sulfate (SO4) in groundwater is derived mainly from gypsum, pyrite, and other

rocks tha t conta in su lfur comp oun ds. At cer tain concentr at ion s, su lfate gives water a

bi t ter tas t e , and a t higher concentra t ions i t acts as a laxat ive. Su lfate , when combined

with calcium , a lso contr ibutes to scal ing in s team boilers .

Iron an d, to a lesser extent , m an ganese are a dded to grou n dwater by most soils an d

rocks. Excessive amoun ts of iron an d ma ngan ese are un desirable in water us ed for

laundry, food processing, dyeing, bleaching, ice manufacturing, brewing, and certain

other indu str ial processes .

Flu oride is foun d in some sedimenta ry and igneou s rocks, bu t is n ot widesprea d. At

cer tain concentr at ions , it redu ces tooth d ecay, bu t at h igher concent rat ions , it cau ses

mottling of tooth en am el.

Man-Made Contamination

Man -ma de, or anth ropogenic, sour ces of grou nd water conta minat ion are as sociated

with ma ny a spects of everyday life. Althou gh ind u str ial incidents involving a sp ill of

"toxic" material are the most obvious sources of potential groundwater pollution,

ordina ry activities s u ch as fi ll in g a gas tan k or over-ferti lizing a lawn can lead to

grou n dwater degradat ion as well. Even less obvious are cau ses th at d o not involve

ha nd ling a toxic mater ial . For exam ple, overpu mping a well in a coastal area can draw

sal twater into freshwater a reas u sed to su pply dr inking water .

Many sources of groundwater contamination involve the release of a polluting

su bsta nce onto or ju st below the groun d su rface. These "surf icial" sour ces include

u nd ergroun d fuel or chem ical s torage tan ks, lagoons , land fills , th e ap pl icat ion of

pest icides to agr icul tur al lan d, sept ic systems , an d even r ainwater ru noff. A sm all

am oun t of a su bst an ce spilled can go a lon g way. For exam ple, a su rface spill of 1 gal

of gasolin e can conta min ate 1 sq m i of a 4 0-ft th ick a qu ifer to levels the U.S.

En vironm ent al Protection Agency (EPA) sta tes are u n su itable for cons u m ption. Figu re

2.5 il lu str ates typical sou rces of ma n-m ad e conta m ination. Atta chm ent IV of Section 4

contains a lis t of common sou rces at indu str ial s i tes .

Nu merou s types of su bsta nces can contam ina te grou nd water . Organic, inorganic, an d

biological su bsta nces al l ha ve the potent ial to degrad e grou nd water qua lity. Common

organ ic contam ina nts include d egreasing solvents (su ch a s t r ichloroethylene an d

tetrachloroethylene, commonly ass ociated with ind u str ial s ites) and petroleum

hydrocarbons (benzene an d tolu ene, common ly associated with gas s tat ions ). Lead

(leaded gas oline) an d oth er m etals an d ch lor ides ( road sa lt a nd sal twater int ru sion) are

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examples of inorganic conta minan ts . San itary wastes can be sou rces of biological

pollutan ts su ch as viru ses an d bacter ia . Drink ing water with h igh bacter ia

concentrations can lead to i l lness, including digestive tract disorders.

CONTAMINANT SOURCES

Contam ina nt sour ces can be clas si fied a ccording to their geometry as point , line, or

diffuse. A poin t s ou rce is on e in which th e contam inat ion or igina tes f rom a s ingle

location (e.g., a sin gle, sm all sp ill). A line source i s one in which contam ina t ion

originat es over the length of a l ine (e.g., a pipeline leaking along a 1 50 -ft length ). A

diffuse s ource (also known a s a non point s ource) is sp read over a wide area th at m ay

not be clear ly defined. Based on the was te ma na gement pra ct ice, these different types

of sour ces m ay also involve cont inu ous releases or discharges of contam ina nts or

sporadic releases to th e groun dwater .

To contam in ate th e groun dwater , su bsta nces general ly mu st f ilter thr ough s oil or

sediments down to the water table . The a bi li ty of the contam inan t to migrate th rough

these surficial materials is influenced by the physical characteristics of the soil and the

propert ies of the conta minan t . Some potent ial contam ina nts ph ysical ly bind to soil

par t icles an d fai l to reach th e grou nd water below. Most conta mina nts int rodu ced at th e

su rface are tran sport ed to th e water ta ble by the in filtra tion of precipitation . The

inf il t rat ing water dissolves th e conta mina nt a nd t ran sports i t thr ough th e soil medium .

(Dissolved contaminants are called solutes .) Th erefore, th e poten tial conta m inan t 's

solub ili ty in water is importa n t. Oth er in fluen tial properties in clude th e ph ysical sta te

of th e su bs tan ce (e.g., solid, l iqu id, or gas), its electrical charge (if an y), an d its ch emical

sta bili ty or persisten ce.

Once man-made contaminat ion reaches groundwater , the movement is governed by

several factors .

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CONTAMINANT TRANSPORT

After contam ina nts enter th e groun dwater flow system, they tend to form plum es of

contam in ated water extending downgrad ient f rom th e source (see Figure 2.6). The

solu te concen trat ions tend to be highest c losest to the sou rce, and decrease to a

min imu m level away from th e sour ce in th e direction of groun dwat er flow. Many factors

inf lu ence the sh ape an d s ize of a plum e:

• G eom e tr y a n d c on t in u i ty of s ou r c e.

• Typ e a n d c on c en t r a tion of t h e p ollu t a n t .

• Loca l ge ology.

• Gr ou n d wa t er flow s ys tem .

• Pr es en c e of p u m p in g w ells .

A plume from a point source tends to be long and narrow if the groundwater is flowing

relatively rap idly, bu t if i t is flowin g slowly, th e cont am inan t will sprea d ou t or diffu se t o

form a wider plum e. The sh ape an d s ize of a plum e can ch an ge su bsta nt ially in direct

respons e to the add it ion of wastes to the system, cessa t ion of waste disposal , or na tu ral

a t t enua t ion mechanisms .

Advection and Hydrodynamic Dispersion

Groun dwater flow is by far th e ma in t ran sport m echan ism for dissolved contam ina nts

(solutes ) in aqu ifers. Two as pects of groun dwater flow are res pon sible for tran sport ing

solutes . One, advection , cons ists of th e bu lk, str aightforward (lin ear) motion of flowin g

grou n dwater . In a dvect ion, solut es are carr ied at an average rate equal to the average

linea r velocity of th e water. Th e second as pect of groun dwat er flow, called

hydrodynamic dispersion , cau ses solutes to be spread out in var ious d irect ions a way

from t he p ath of linear (advective) flow. Dispersion is cau sed by mech an ical mixing

du ring advective flow an d m olecu lar diffu sion du e to the th erma l-kinetic energy of th e

solute pa rticles. Th e net effect of dispers ion is to dilu te the s olu tes.

Attenuation Mechanisms

Several processes in groun dwater f low systems retard the migrat ion of contam ina nt

plum es. Attenu at ion m echan ism s inclu de filt rat ion, sorpt ion, chemical processes ,

biological degrada t ion, a nd di lu t ion. These p rocesses help to remove conta minan ts

from groun dwater . How quickly contam ina nt concentrat ions are reduced depend s on

the type of conta mina nt a nd the local

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7/29/2019 Manual m Amb 2-1


hydr ogeologic setting. The processes a cting to redu ce cont am ination ten d to localize

grou n dwater pollu t ion n ear i ts source.

Filtration is an a t t enu a t ion mechan ism th a t i s mos t impor tant a t the su r face , where

contam in an ts firs t enter the grou nd . Filt rat ion by the soil colum n rem oves su spen ded

ma ter ials . Within th e groun dwater system, f ilt rat ion can also remove par t iculates of

i ron and manganese and chemical precipi tates .

Sorption is a very imp ortant a t tenu at ion m echan ism for reducing groun dwater

contam in at ion. With the general except ions of chlor ide, ni t rate , an d su lfate , ma ny

contaminants can be adsorbed under the r ight physical and chemical condi t ions.

Clays, m etal lic oxides an d h ydroxides, a n d organic m at ter , wh ich occu r n atu ral ly within

the groun dwater flow system, can al l ads orb conta minan ts f rom groun dwater .

Chem ical process es serving as a t t enu a t ion m echan isms to reduce groun dwater

conta min ation includ e precipitation, oxidation, volatil ization, a n d ra dioactive decay.

Chemical precipi tat ion is a m ajor at ten u at ion m echan ism in ar id clima tes . Oxidat ion of

organ ic mat ter is an important m echan ism in the zone above the water table .

Volatil ization is importa nt in r eactions involving n itrate an d su lfate. Radioactive decay

applies only to radioactive contaminants.

Biolog ica l d egra d ation breaks down organ ic pollutan ts th rough th e act ions of na tu ral ly

occu rr ing microorgan ism s su ch as ba cter ia , viru ses , an d fun gi. Depending on th e

oxida tion state of th e aquifer, ana erobic or aerobic biodegra da tion can occur. The

processes u sed by th e microorgan ism s ar e not always well u n derstood, and incomp lete

decomposit ion can produ ce interm ediate by-products . The f ina l products of organ ic-

contam ina nt degrada t ion are carbon d ioxide an d water .

FURTHER INFORMATION

A glossa ry of comm only us ed geological an d hydr ogeological term s is includ ed in

Appen dix A. App end ix E conta ins a l ist of references for fu rth er stu dy.

manual m amb 2-1

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