Industrial and Urban Pollution of Groundwater

8/3/2019 Industrial and Urban Pollution of Groundwater

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1 UK Ground water Forum

The risk from industrial chemicals

The exponential grow th of th e petrochemical andpharmaceutical industries in the 1950s introduceda w ide range of comp lex organic wastes into theurban environm ent. A serious risk togroun dw ater quality has arisen from acciden talspills or leaks from tan ks and pipelines of petroleum p rodu cts, phenols and chlorinatedhyd rocarbon s. Some of these substances aresoluble in water bu t man y are only slightlysoluble. The latter are referred to as non-aqueousphase liquids (or NAPLs) and they are d ividedinto light and dense N APLs accord ing to wh etherthey are less or more d ense than water. LightNAPLs are mainly petroleum pr odu cts and th edense variety include th e chlorinatedhyd rocarbons widely used as ind ustrial solvents.The concentration of these compou nd s allowed indrinking w ater is in the p arts per billion range.Many a re sufficiently soluble to m ake it possibleto exceed th ese very low limits.

When a N APL is spilled on t he sur face of asandstone, it infiltrates slowly dow n thr ough thesaturated zone m oving from pore to pore once thesaturation of the h ydrocarbon exceeds a th resholdknown as the residu al saturation. The volum e of the contaminant that is m obile steadily decreasesbecause some is retained in each pore at theresidu al saturation. Thu s with a small spill thecontam inant m ay be retained entirely in theunsatu rated zone once all the spill is at residu alsaturation in the rocks. With a larger orcontinuous spill the NAPL will eventually reachthe capillary fringe. As it m oves down throu ghthe un saturated zone is also spread s laterally,

because capillary forces as well as grav ity controlits migration. In fractured rocks progress may bemu ch more rapid w ith the contaminan t followingthe most permeable route.

When t he N APL reaches the capillary fringe, if itis a light N APL, it will accum ulate nea r the topand flow in th e direction of the hydr aulic gradient

Industrial and urban pollution UK Groundwater Forum of groundwater

Pollutio n of the Chalk by t he solv ent t etrachloroethyl ene (a dense NAPL). Migratio n of t he pollut ant below the w at er t able is retarded by a lay er of chalk of low permeabilit y. Below this lay er the pollut ant is m igrating in t he fractures in t he aquifer and collecting at t he base. Pollut ant dis solv ed in groundw at er is migrat ing through t he aquifer in the direction o f groundw at er flow .




once a critical thickness is attained . Thecontaminant continues to spread u ntil the entirespill is at r esidu al saturation.

When a dense N APL reaches the capillary fringe,dow nw ard flow continues as before once theresidu al saturat ion of each por e is exceeded . Thiscontinues u ntil it reaches the base of the aqu ifer,again un til the entire spill is at residu al saturation.The direction of flow in th is case may n ot coincidewith the hydr aulic grad ient.

Pollution of an aqu ifer by N APLs can increase byslow d issolution in groun dw ater. The resultingcontam inant plum e can be much larger than thespread of the spill itself and , because of the slowrate of dissolution, can per sist as a source of pollution for d ecades.

Light NAPLs are mor e easily controlled as theyfloat on the water tab le and occur at relativelyshallow d epths. Dense NAPLs move und er the

influence of their d ensity and can penetrate toconsiderable depths. Once an aquifer iscontam inated it is likely to remain so as rem edialmeasures tend to be ineffective.

A single spill has the poten tial to conta min ate avery large volu me of w ater to levels in excess of the low concentration acceptable in d rinkingwater. The wide distribution of petrol stationsand extensive networks of p ipelines carryingpetroleum prod ucts illustrates the potential risk togroundw ater quality.

A spill of the solvent tetr achloroeth ylene, a den seNA PL, at a leather -processing factory inCambridgeshire, polluted th e Chalk to a depth of 50 m an d extended for at least 2 km in thedirection of groun dw ater flow. A m ajor pu blicwater supp ly pu mp ing station becamecontam inated . Because of the scale of the sp ill andthe gradu al dissolution of the contaminan t in thegroun dw ater, the contamination is likely topersist for m any years.

At present the water indu stry must combat thethreat by treating polluted w ater or replacing the

source — both ar e very expensive option s toovercome a p roblem largely caused bycarelessness.

Prevention is cheaper than cure

The Ground water Pr otection Policy of theenvironment agencies sets out a framework of guidan ce within w hich everyone can ensuregroun dw ater resources are safeguard ed for thefuture. One advantage is that developm entswh ich pose a risk to the quality of ground watercan be assessed at the p lanning stage and if

necessary modified or even rejected.

An examp le that poses a p articular risk is a petrolfilling station on an aqu ifer such as the Chalk,

par ticularly if it is close to a pu blic sup p lyborehole.

The very fractured natur e of the chalk, and itsresulting high p ermeability, provides a read ypassage for the flow of pollutants from thegroun d sur face to the water table. In thissituation a petrol station on the Ch alk und oubted ly poses a considerable risk togroun dw ater quality. It is unacceptable in Zone Iof the Source Protection Z one (see ‘Vulnerableaquifers need pr otecting’) and should bediscouraged in Zones II and III around a pu blicsupp ly borehole.

Modern p etrol stations should incorporate amulti-layered system to contain any leakage fromtheir storage tan ks or the forecourt. These includ edou ble-skinned tan ks in a concrete vault linedwith synthetic polymer m embranes, andcomp acted clay. In t he short-term theseprecautions do afford an adequ ate safeguard , butcomponents deteriorate, concrete cracks, syntheticmembr ane may be breached, and , as the integrityof such a system cannot be mon itored in apractical man ner, it does not provide a reliableeffective long-term barrier ag ainst leakage.

What may be imm ediately at risk from a p etrolstation is a source of public supply p rovidingperhap s 5 million litres of water p er d ay for some30 000 peop le. The Drinking Water Stand ard fordissolved or emu lsified hyd rocarbons is only10 microgram s per litre (that is 1 par t per 100

million parts of w ater). Once a fine-grainedaqu ifer such as the Chalk is contamin ated it iswell nigh imp ossible to clean it up to dr inkingwater stand ards. The cost of a replacement w atersup ply could be at least £5 million. The carefulassessment of p lanning ap plications for thestorage of chemicals in a Groundwater ProtectionZone is, therefore, a wise precaution .

The dispo sal of radioactive waste

Much concern h as been expressed abou t the safedisp osal of rad ioactive w aste. For the gener al

pu blic it has become an em otive subject as thewaste remains rad ioactive for m any years andunless prop er safety pr ecautions are takenradiation can be a risk to health.

Small amou nts of very low-level rad ioactivewaste have been d isposed of in land fills, but mostlow-level waste is dep osited in concrete vaultsand trenches in glacial d eposits at Drigg, nearSellafield in Cu mbr ia.

A favoured preference for the disposal of intermediate-level waste, and also longer-lived

low-level waste, w hich cannot be disp osal of atDrigg, was deep d isposal in an und ergroundrepository, constructed at a dep th of about 1 kmin very imp ermeable rocks.




How ever, the plans to develop th is facilitybeneath Sellafield were n ot app roved by the localcouncil.

The objective was to tak e ad vant age of the factthat w ater in such rocks is extremely old, very

saline and is m oving exceedingly slowly. Toguard against the risk of contaminating thebiosph ere, the concept behind such deep bu rialwas to p rovide a containment facility w ith severalbarriers to prevent the m igration of the rad ionu clides. This is referred to as the mu lti-barriercontainmen t concept.

The wastes, in steel or concrete containers wer e tobe placed in excavated chamber s in the rock wh ich w ere then to be backfilled w ith a cement-based m aterial. The backfill would have had ahigh surface area to absorb radio nu clides, and itwou ld have m aintained alkaline cond itions in thepore waters and thereby supp ressed th e solubilityof imp ortan t rad io nuclides. The host rock itself wou ld have provid ed a final barrier with a longpathw ay and time of travel in slowly movinggroun dw ater to the biosphere. Any radio nu clidesthat entered the ground water, slowly flowingthrough the rocks, would have been retarded by

adsorp tion and diffusion into the r ock matrix. Theconcept mak es use of engin eering, chem ical andnatur al barriers to achieve long-term containm ent.

Leachates from landfills

Each year over 150 million ton nes of solid w astefrom d omestic, commercial and indu strial sourcesare disp osed of in land fills. The waste ran gesfrom inert waste with a low p olluting p otential tohazar do us chem ical wastes. Land filling is oftenthe cheapest disposal option.

Mult i-barrier contai nment concept for the disposal of low -level and intermediat e-level radioa ctiv e w ast es.

Classification of radioactive w aste

• Low-level waste, mainly discardedclothing and wrap ping m aterials. Itcompr ises abou t 90% of the total volum eof radioactive waste and represents onlya small risk.

•

Intermediate-level waste comprisingmetal fuel cladd ing, reactor comp onentsand chemical sledges. It form s 10% of thetotal waste volum e and h as a longlifespan of radioactive decay. This wastemu st be shielded from the environmentand an option is disposal in a d eeprepository.

• High -level w aste arising fromrepr ocessing of nu clear fuel. It comp risesonly 0.1% of the total bu t contains 95% of the rad ioactivity. The cur rent p olicy is tostore this w aste at the sur face in avitrified form, un til its temp erature h asdropped sufficiently to allow safedisposal.




Solid w astes pose a threat to groun dw ater qualitythrough the formation of polluting liquids knownas leachates.

These form as water p ercolates through t he w aste,dissolving soluble comp ound s and thedegrad ation prod ucts of chemical andbiochemical reactions that t ake place in the w aste.Dom estic solid wa ste gives rise to a very p ollutingleachate. It d ecomp oses und er anaerobicconditions after a brief aerobic stage of a fewmonth s; a final stage leads to the p rodu ction of methane, by which time the polluting strength of the leachate is reduced. The entire decompositionpr ocess can take decad es, the rate being verymu ch a function of the amount of water that cangain access to the w aste.

There are thou sand s of land fills in the UK. Manyare old or d isused and now classified as

‘contam inated land ’. Mod ern landfills arecarefully constructed so that th e liquid w asteprod ucts are contained. In the past it wasconsidered accepta ble to allow leachates to seepfrom the base of the waste into und erlying rockswhere they were diluted by the ground water andgradually dispersed through the groun d.Percolation throu gh rock d oes improve theirpolluting strength bu t such pr ocesses are notentirely reliable and any risk of pollutinggroun dw ater in this manner is not nowacceptable.

Instead lan dfills are lined w ith clay and flexiblesynthetic mem branes, intend ed to preventleachate escaping and contaminatinggroundwater. The leachate is drained to collecting

sump s for subsequent treatment. Methane is alsocollected and either vented to the atm osphere, orused for heating, even the generation of electricity.

The current disposal practice is designed to createvery comp act dense w aste wh ich effectivelyredu ces its contact with w ater.

Furthermore, the w aste is covered w ith a layer of clay to p revent, or at least red uce, the ingress of surface water . This pra ctice red uces the rate of decom position. It ma y be decad es before leachateand gas are prod uced, especially in the large anddeep sites that are n ow favour ed. Recent viewsare that water should n ot be excluded and th atideally land fills should be very efficientbioreactors with high rates of leachate formationand accelerated rates of decom p osition. A land fillshould reach a final stable unpolluting state

within ou t about 30 years. This could mean that itis necessary to irrigate the w aste with w ater orrecycle the leachate to encour age decomp osition.

Landfill operators are requ ired to hav e a licencefrom one of the environment agencies and toadhere to w aste managemen t licensingregulations. Landfills must be designed andconstructed to high engineering standard s andensure containm ent of waste produ cts andthereby prov ide long-term protection of theenvironment. How ever, although much is know nabout the p rocesses in a land fill, monitoring

facilities aroun d sites and t he assessment of theperformance of sites are relatively un developed.There are no well-defined stand ards of performance.

Cross-sect ion of a mod ern landfil l. The design incorporat es a sy nthet ic membrane liner (about 2.5 millimet res thick) on a layer of low -permeabilit y clay (about 1 met re thick). The leachat e is drained to a sump by a net w ork of pipes in a permeable drainage lay er. Any gas generat ed is collected by means of a syst em of pipes. (Not draw n to scale).




The efficacy of the long-term containm ent of wastes has yet to be dem onstrated and the overallrisk to groun dw ater quality remains uncertain.How ever, althou gh land fills are potential threatsto ground water qu ality, any pollution that occursis localised and quite d issimilar to the m orewidespread pollution that arises from agricultureand from urban areas. Nevertheless, reducing thepolluting imp act of land fills remains a p rimeconsideration.

Contamin ated land and urban pollut ionContaminated land and urban p ollution arelegacies of the Industrial Revolution and thesubsequent ind ustrial expansion. For over 200years, indu strial wastes have been disposed of,and sp illed in and near urban areas, and m anycontain potential ground water contaminants.Initially they were mainly mining and found rywastes, essentially inorganic compou nd s, buteventually they included tars, phenolic wastes,oils and , more recently, comp lex synth etic organ iccomp ound s. These and similar substances havecontam inated large areas of land particularly inurban areas but also aroun d m ines, militaryinstallations and old landfills.

Land contaminated in this mann er can give rise toa range of liquid and gaseous pollutants, and themore soluble and mobile infiltrate to the w atertable. Below m any ind ustr ial sites there areplum es of contaminated gr ound water slowlymoving dow n the h ydraulic gradient.

It has been estimated that over 500 squarekilometres of conta mina ted lan d exist in the UK,distribu ted ov er mor e than 50 000 sites, althou ghonly some pose a threat to the qu ality of groun dw ater. A particular risk arises wh en sites

Laying a liner of synthet ic membrane for a newlandfill.

Migrati on of a plume of leachat e from a landfi ll. The high chloride concentrat ions in t he groundw at er (theunit s are milligrams per litre) show the extent of the plume. Landfills are now designed so t hat l eachat es are

contai ned in t he structure and collected for appropriate d isposal . Contaminat ion of underly ing aquifers should not occur.




are redeveloped, as this disturbs and mayremobilise pollutants.

Land is contaminated wh en substances arepresent at concentrations that could be har mful tohum ans, animals or the environment as a whole.To pose a risk there mu st be a source of contam ination, a pathw ay that allows the

contam inant to m igrate, and a specific location (ortarget) wh ich may be affected. To pu t this in agroun dw ater context w ith a typ ical examp le, oldgasworks p rodu ced coal, or ‘town ’, gas un til themid-1960s, contaminating t he grou nd und erlyingthe works with comp ound s including coal tar,phen ols, cyanides, heavy metals and sulph urcomp ound s. These substances were dissolved byinfiltrating water and the ground water becamecontam inated over a much greater area. If thegroun dw ater flowed to a borehole or well used asa source of drinking w ater, then the risk sequencewas comp leted.

The decontamination of such a site may involvepu mp ing and treating the ground water, as well asthe rem oval of the contam inated soil to land fills,or the trea tme nt of the soil on site by one of therem edial meth od s available. Such landreclamations are obviously extremely costly.

The scale and natu re of the problem of contaminated land in urban areas and aroundindu strial comp lexes remains u ncertain.Currently this is being assessed by theenvironment agencies. There is not a nationalregister of contaminated land but local authoritiesare comp iling their own registers, and h avepow ers to investigate potential problems and

require remediation wh ere a risk to theenvironment or pu blic health can bedem onstrated. The onus for cleaning-upcontam inated land, how ever, is with the own er.

Generally it can be assumed that ur ban areas arecentres of groun dw ater pollution from ind ustrialzones, railways, and possibly leaking sewers.

Stud ies of the quality of groun dw ater below th ecities of Birmingham and Coventry revealed th atpollution by organic and inorgan ic compou nd swas extensive. The p redom inant organicpollutants were chlorinated h ydrocarbonsolvents. There was a close correlation betw eenland u se and the pollutants identified.Widespread contamination of ground water bysolvents in the Chalk below Luton has also beenrecorded.

A survey of the extent of groun d water p ollutionin England and Wales, by the Environment

Agency, revealed that th e m ain sources of pollution were land fills, the chemical and metalprocessing indu stries, and gas w orks, powerstations and petrol service stations. The pollutantsmost frequently recorded were metals(particularly arsenic, copp er and chromium ),organic comp ound s and landfill leachates. TheChalk and the Triassic sandstones in the Mid landsand n orth-west England w ere the aquifersparticularly affected.

Usually the effect on grou nd wat er is localisednear the site in qu estion, although in u rban areaswhere there are m any p otential sources of pollution, contamination is widespread , as inCoventry and Birmingham . Cleaning-up

The frequency t hat part icular cont ami nants occur in groundw at er in England and Wales.The dat a are from a survey by t he Environment Agency of groundw at er pollutio n.




contam inated groun dw ater is of growing interestbut the cost of clean-up , is set against the ben efitsthat accrue, is also an imp ortan t consideration . Insome situations, for examp le where thegroun dw ater is not likely to be used for drinking,less stringent levels of ‘clean-u p’ may beapp ropr iate. The question that mu st be answeredis: does the contam ination p oise a poten tial risk tohum an health, or the environment, on the basis of current and anticipated u se of the site or area?

Remedies for pollutionIf groundwater is contaminated by a relativelysmall-scale source of pollution such as a spill of hydrocarbons, the question is: how can thequality be restored or at least improved to anacceptable standard ?

The first concern is to determine the distributionof the contaminant in th e aquifer, whether it is inthe unsatur ated zone or the saturated zone orboth, and w hether it is entirely in solution or thereis a non-aqueous ph ase. Answers to thesequestions will suggest a possible remediationstrategy as show n below.

Despite the ava ilability of these techn iques,restoring an a qu ifer to its original pristine state isdifficult if not imp ossible. When a contam inatedaquifer is pu mp ed, the groun dw ater flowspreferentially throu gh the m ore permeablehorizons and consequently these are cleanedrelatively easily and quickly. However,contam inants that have p enetrated into lessperm eable horizons and the smaller pore spacesmay be virtually unaffected. When pum ping

stops, contaminan ts from th ese zones diffuse into

the perm eable zones that have been cleared. Themore p ermeable fractures in th e Chalk representonly 1 or 2% of the total rock volum e wh ile therelatively immobile water in the m atrix represents25 to 40%; in th is situation t he cleaning p rocesscan take a very long time. Similarly, if nu trientsare injected to encourage biodegrad ation theymay penetrate only slowly into the less permeablehorizons.

The difficulty of cleaning an a quifer emp hasisesthe need to minimise all forms of pollution.

Where it does occur, the problem shou ld betackled at or as near the sou rce as possible and assoon as possible after the event. It mu st beaccepted that with existing technology,groun dw ater quality often cannot be restoredwithin a r easonable time and at a reasonable cost.Nevertheless, wherever p ollution occurs in anaquifer it should be contained and treatmentinitiated, bearing in m ind the u se made of thegroun dw ater, any h ealth risk that exists and cost.

The contami natio n of groundw at er below an urban area in t he Midlands. The map show s t he present and est imat ed fut ure contam inati on of groundw at er by chlorinated hyd rocarbon solvents.

On-sit e treatment of groundw at er pumped from an aquifer conta minated by chlorinated hydrocarbons.




Mine drainage

Coal and m etalliferous mines can be hu nd reds of metres deep, with the w orkings well below thenatur al water table. Consequently, deep mining isonly possible if mines are drained by p um ping.At the time of maximum coal outp ut from m inesin England and Wales some 450 million cubicmetres of water w ere pum ped each year fordr ainage p ur poses. The collieries of South Walesand Scotland were p articularly wet, pu mp ing 8and 10 cubic metres of water respectively, pertonne of coal mined, although th e averagedrainage-output ratio for collieries in Englandand Wales as a whole was 2 cubic metres pertonn e of coal.

Coal seams and mu dstones in the Coal Measurescontain p yrite (ferrous sulphide) w hich wassubject to oxid ation by air as the wat er table waslowered, the process being supp orted by sulphu r-

oxidising bacteria. As a result, water drainingfrom coal mines can be very acidic, w ith p Hvalues of 2 or 3, due to sulphu ric acid, and highconcentrations of sulphate and iron, as well asother m etals, in solution . Similar w aters form inmetalliferous m ines and in m ine wastes.

The discharge of such water s to rivers isdetr iment al. The iron precipitates as ferrichyd roxide w hen the p H r ises above 6, followingreactions with the river sediments and mixingwith alkaline waters in rivers, and w hen bacterialoxidation of the ferrous compou nd s occurs. The

iron precipitate discolours the rivers leaving

unsightly ochrous dep osits and the oxid ationprocess leads to th e dep letion of the oxygen in theriver water.

Ferruginou s waters discharging from disusedmines have been a pr oblem for decades, but w iththe closure of so man y coalmines, beginning inthe 1950s and cont inu ing into th e 1990s, theproblem h as been accentuated . As mines closed,pum ping stopp ed or was reduced andgroun dw ater levels recovered. The rate of recovery dep ends very mu ch on the rate of flowinto the w orkings and their extent. In some casescomplete recovery m ay take several decad es. Aswell as contaminating rivers, rising acidicferruginous groundw ater may in the futurecontam inate aquifers overlying t he Coal Measuressuch as the Perm ian Basal Sand s, the Magn esianLimestone and the Permo-Triassic sandstones.The rise of water levels can also lead tosubsidence as a result of the weakening and

eventual collapse of strata left to su pp ort th e roof in shallow workings. The corrosion of engineeringstructures that contain Portland Cement and theflooding of landfills are other p roblems.

The quality of the discharge from disused minesimproves grad ually with time as the risinggroun dw ater flushes the acidic oxidationprod ucts out of the Coal Measures, but theprocess can take man y years.

Increasingly w idespread pollution by su ch acidicferruginous w aters seems to be inevitable un less

expensive treatment m ethods are introdu ced orpu mp ing from some m ines is continued to controlthe d ischar ges. The circulation of contam inatedferruginou s w aters throu gh artificiallyconstructed w etlands, which impr oves theirqu ality, is being investigated as a relatively cheapsolution to the p roblem.

Responsibility for pollution by acid mine-watersis very un clear. Mine owners wh o perm itpollution from abandoned mines are givenindemn ity un der the Water Resources Act of 1991and, in Scotland, the Control of Pollution Act of

1974. The Environ men t Act 1995 rem oved thisdefence but only for mines abandoned after 1999.This leaves the legal responsibility and theproblem unresolved.

The rebound of groundw ater levels in coalfields

The recovery of groundwater levels in coalfieldsthat are no longer mined h as caused seriousenvironmental problems in many areas andpotential future problems in others. The coalfieldsin Scotland afford n um erou s examples, a case inpoint being the Dalqu harran Colliery in Ayr shire,

wh ere pum ping for drainage purp oses ceased in1977. Ground water levels recovered and waterbegan d ischar ging from th e mine in 1979 at rates

Contaminat ion of the River Esk in East Lothian by ferruginous groundw at er discharging from an abandoned coal mine.




rising to 13 Ml/ d. The water flowed into theRiver Girvan . As it contain ed 1200 mg/ l of iron,100 mg/ l of aluminium and 6000 mg/ l of sulp hate, the effect on the ecology of the river w asdevastating. The problem was ameliorated byadd ing lime to the outflow and controlling therate so as to link it to the flow of the river , therebyensuring dilution.

The mine workings in the Du rham Coalfield areextensively interconn ected. While the collierieswere operating, ground water levels weremaintained som e 150 metres below th e groundsurface by pu mp ing almost 100 Ml/ d from nine

pu mp ing shafts. The quality of the water wasquite good and most was d ischarged into theRiver Wear p rovid ing n early 50% of the flow indry spells. Once this pu mp ing stops, the risinggroun dw ater will dissolve the oxid ation prod uctsof pyrite in the Coal Measu res to give an acidicwater, with large concentrations of metal ions insolution, wh ich will discharge u ltimately into the

river system. The Permian aquifers that overliethe Coal Measu res are also likely to becontam inated. Comp lete recovery of groun dw ater levels in the Durh am Coalfield maytake 30 or 40 years.

British Geological Survey. © NERC. All rights reserved [IPR/47-4]

Industrial and Urban Pollution of Groundwater

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