Pacific Science (1985), vol. 39, no. 2© 1985 by the University of Hawaii Press. All rights reserved

Uses and Abuses of Wastewater Injection Wells in Hawaii!

FRANK L. PETERSON2 and JUNE A. OBERDORFER3

ABSTRACT: During the past two decades in Hawaii, more than 500 injectionwells for the disposal of domestic sewage wastewater have been constructed andoperated. Thus far , contamination of potable groundwater supplies has not beena problem. Many of the injection wells, however, have not performed as de­signed , and aquifer clogging and reduced injection capacity have producednumerous well failures resulting in public health, legal, and financial problems.Factors most commonly responsible for the well problems have been unfavorablehydrogeology, underdesign of injection well capacity, poor effluent quality, andlack of injection well maintenance. Detailed study ofclogging mechanisms in theimmediate vicinity of injection wells suggests that binding of pore spaces bynitrogen gas is the most important cause of aquifer clogging. Other cloggingmechanisms also operating are filtration of solid particles and growth ofmicroorganisms.

THE HAWAIIAN ISLANDS ARE principally de­pendent on groundwater for potable watersupplies. Consequently, the disposal of liquidwastes into the subsurface is of great concern.The principal mode of groundwater occur­rence is the basal (or Ghyben-Herzberg) lensof fresh water overlying and displacing thedenser saline water. The basal groundwaterbody is generally thickest and freshest whererecharge (i.e., rainfall) is greatest, which isgenerally in the interior portions of the islands .Along the coastal margins of the islands,groundwater bodies are generally thinner andmore saline . The predominant aquifers arehighly permeable basaltic lava flows. How­ever, in the coastal portions of the olderislands, especially Oahu and Kauai, less per­meable marine and alluvial sediments, com­monly referred to as caprock, often occur andmay confine fresh basal water beneath them .The caprock materials may also contain somefresh groundwater, but more commonly con­tain brackish water.

I Manuscript accepted 1 November 1984.2 Un iversity of Hawaii at Manoa, Depa rtment of

Geology and Geophysics, and Water Resources ResearchCenter, Honolulu, Hawaii 96822.

3 San Jose State University, Department of Geology,San Jose , Californ ia 95192.

Because the Hawaiian Islands are sur­rounded by the Pacific Ocean and the vastmajority of the population lives in the coastalregion , disposal of municipal wastewaters hasbeen achieved mainly by ocean outfalls in theurban sewered areas and by cesspools in therural unsewered regions . During the last twodecades, however, numerous hotels, apart­ments, and condominiums have been con­structed in outlying unsewered areas, generallyalong the coast. These new facilities have pro­duced volumes of sewage that for the mostpart are too great for cesspool disposal, buttoo small for economic ocean outfall disposal.As a result, the use of injection wells for sub­surface disposal has proliferated, often withless than satisfactory results (Figures 1 and 2).

HAWAIIAN INJECTION WELLS

At present there are more than 250 injectionfacilities that utilize over 500 injection wells inthe state. These wells are used for a variety ofindustrial and domestic wastes, but the major­ity are for the dispo sal of treated sewage efflu­ent. Figure 3 shows the generalized location ofinjection well facilities in the State of Hawaii.

Most wells are privately owned and oper­ated and are characterized by shallow depth

230

Wastewater Injection Wells-PETERSON AND OBERDORFER 231

FIGURE I. Overflow of sewage effluent from clogged injection wells, Ewa Beach, Oahu.

(usually less than 30m), small diameter (0.10mbeing the most common), and injection ratesof only a few hundred liters per minute. Inaddition, there are several municipal injectionwell facilities on Oahu and Maui. The wells atthese facilities are generally deeper and largerthan the private installations, and typicallyinject several hundred thousand to a fewmillion liters per day of wastewater.

Most of the injection wells in Hawaii, es­pecially those for disposal of treated sewageeffluent, are located in the coastal regionwhere the receiving waters are brackish orcompletely saline. In this environment thegroundwater table usually lies only a fewmeters below the ground surface; therefore,water table fluctuations resulting from oceantides and storms and seasonal changes ingroundwater recharge often significantly af­fect injection well performance. The receivingformations are generally sedimentary caprockmaterials, but in some regions, especially onHawaii Island, the receiving formations arelava flows. Figures 4A and 4B show a hydro-

geologic cross section and a plan view ofwastewater injection into a typical coastalaquifer environment.

INJECTION WELL PROBLEMS

Wastewater injection poses two distinctlydifferent types of potential problems in theHawaiian environment. If the injectant mi­grates too far from the injection wells withoutsufficient dilution by the resident ground­water, contamination of potable groundwatersupplies and the shallow nearshore coastalwaters may result. Contamination of freshgroundwater bodies by injected wastewaterhas been investigated in detail by Peterson,Williams, and Wheatcraft (1978) and Wheat­craft and Peterson (1979), and is not known tobe a significant problem at the present time.Fortunately, because virtually all wastewaterinjection is restricted to coastal areas wherethe groundwater is generally brackish orsaline, freshwater aquifers have not been

232 PACIFIC SCIENCE, Volume 39, April 1985

F IGU RE 2. Compressed air used to unclog injection well, Ewa Beach, Oahu.

threatened. The Honolulu Board of WaterSupply (1982) and the Hawaii State Depart­ment of Health (1984) have set stringent con­trols on the placement of injection wells(Figure 5). Wastewater injection is generallyallowed only in those areas where the chlo-

ride content in the groundwater exceeds5000 mg/liter , Furthermore, in areas wherebasaltic aquifers containing potable waterunderlie sedimentary caprock, injection intothe caprock is permitted only where at least15m of nonpermeable material separates the

Wastewater Injection Wells-PETERSON AND OBERDORFER 233

OKAUAIalHAU .

o,

HAWAIIAN ISLANDS

~OA:IHU~ MOLOKAI

~

"",CV DCJ

KAHOOLAWE

FIGURE 3. Generalized locat ion of injection well facilities (stippled areas) on the islands of Kauai , Oahu , Molokai,Maui , and Hawai i.

potable groundwater from the bottom of theinjection wells.

The extent of shallow coastal-water con­tamination is more problematic. Wastewaterinjected into coastal aquifers only a few tensor hundreds of meters from the shore mustdischarge, virtually undiluted, directly intothe coastal waters (Figure 4A, B). The effectsof coastal discharge are primarily a functionof how deep and how disperse the discharge is,with deeper and more disperse discharge hav­ing less impact on shallow nearshore waters.In areas of extensive injection well develop­ment there have been few, if any, complaintsof coastal-water contamination; however, nocomprehensive study has been conducted toevaluate this problem. Clearly, more work isneeded in this area.

A second and more serious problem posedby subsurface waste injection in Hawaii isclogging and rapid reduction of injectioncapacity in the immediate vicinity of the wells(Figures 1and 2). Work by Petty and Peterson(1979) indicates that with the exception of a

very few areas (the most notable being theKona Coast region of Hawaii Island), wellover half ofall Hawaiian wastewater injectionwells have experienced significant cloggingproblems. The problems are manifest at smallprivate facilities as well as at larger municipalplants, and have ranged in severity from slow,gradual loss of injection capacity over manymonths or a few years, to rapid and sometimesalmost complete loss of injection capacity dueto catastrophic events , such as treatment plantfailures. A frequent result ofsevere clogging iswell overflow, where a portion of the effluentdischarges onto the ground near the well head .Public health and aesthetic problems oftenensue, and legal action has resulted in severalinstances.

Given the rather dismal past record of injec­tion well operation, the question must beasked, "Can injection wells be used success­fully in the Hawaiian environment, and ifso, under what conditions?" To answer thesequestions we must understand how and whyclogging occurs .

234

A. Cross Sect ion

Water Table

B ASALT(Fr esh Groundwater )

B. Pl an Vi ew

PACIFIC SCIENCE, Volume 39, April 1985

.:..·....·....·..·.··.:1£:;•.••••·.•.••·••·••••••.•••..••••~••••

Inje cti onWell

~-UnderlyinQ /

'''~;::''''~

-->------

Shoreline

OCE A N

•

FIGURE 4. Wastewater injection into a typical coastal aquifer in Hawaii . A, cross-sectional view; B, plan view ofwastewater movement and coastal discharge.

CAUSES OF CLOGG ING

Virtually all the research done in Hawaiiand elsewhere indicates that some degree ofclogging of injection wells is inevitable , re­gardless of the suitability of the receivingformation, the quality of the injectant, or

the sophistication of the injection operation(e.g., see Ehrlich, Vecchioli, and Ehlke 1977,Harpaz 1971 , Oberdorfer and Peterson 1982,Olsthoorn 1982, Petty and Peterson 1979,Ragone 1977, Rebhun and Schwartz 1968,Vecchioli and Ku 1972, Vecchioli, Ku, andSulam 1980). However, past experience also

Wastewater Injection Wells-PETERSON AND OBERDORFER

Kahuku Pt.

PROTECTEDAQUIF IER

N

~o 4 8 km

I 1

235

FIGURE 5. Proposed underground injection control line for Oahu. No injection wells are allowed inland of thedashed line. After Hawaii State Department of Health (1984).

clearly indicates that the selection offavorableinjection sites, proper injection well operationand maintenance, and effluent quality controlgreatly enhance injection well success.

In their study of Hawaiian wastewater in­jection well problems, Petty and Peterson(1979) determined that several factors werelargely responsible for injection well failures .The most important of these are (1) unfavor­able hydrogeology, (2) underdesign of sus­tainable injection well capacity, (3) poor efflu ­ent qua lity, and (4) lack of proper injectionwell maintenance.

Most commonly, unfavorable hydrogeo­logic conditions result from low-permeabilityreceiving formations. Generally, volcanicrocks comprise the most favorable injectionformations, but in some cases poorly perme­able lavas, especially ponded flows and weath-

ered zones, have experienced severe cloggingproblems. In the caprock , coral reef and reefrubble material are most suitable for injec­tion, with the fine-grained sediments ex­periencing the greatest clogging problems. Anadditional factor ofcritical importance that isoften overlooked in selecting injection wellsites is that virtually all geologic formationsundergo substantial reductions in permea­bility during injection. Thus, formations thatinitia lly have only modest permeability maybe totally unsuitable for wastewater injection.Oberdorfer and Peterson (1982) recommendthat a minimum injection capacity of 100liters/min per well be.required for all Hawaiianwastewater injection sites.

Shallow groundwater tables also contributeto injection well failures . In coastal regions thewater tab les usually are less than 5m below

236 PACIFIC SCIENCE, Volume 39, April '1985

TABLE I

% OF TESTED

m3/sec x 10- 4 gal/min CAPACITY

> 60 > 100 33.330-60 50- 99 2515- 29 25- 49 20

< 15 < 25 *

tion capacity. For example , from Table 1, aninjection test flow rate of 40 x 10- 4 m3/ sectranslates into an injection capacity of only25% of that, or 10 x 10- 4 m3/sec. If theseclogging factors are not recognized and ac­counted for in the design, failure isinevitable.

Inconsistent and often poor-quality efflu­ent , especially at many of the small privateinjection systems, has greatl y accelerated theclogging process . All injected effluent sup­posedly undergoes secondary biological treat­ment, usually some combination of extendedaeration and/or aerobic digestion; however ,high concentrations of suspended solids, 5­day biochemical oxygen demand (BODs),nitrogen compounds, and oil and grease oftenpersist. Table 2 shows the concent rations ofselected constituents in wastewater at severalOahu injection well sites. As can be seen fromthis table, a significant portion of the sitesdid not meet the Environmental ProtectionAgency standards for secondary effluent of amaximum of 30 mg/liter of suspended solidsand BODs. Most of the sites not meeting thesestandards have experienced severe cloggingproblems, including well overflow . Althoughclogging of most injection wells appears to beinevitable, in many cases the adverse effects ofclogging can be significantly reduced and theoverall lifetime of the well lengthened con­siderably if appropriate well maintenanceand rehabilitation practices are followed . InHawaii, regular injection well maintenancehas been only rarely practiced, and well re­habilitation measures often have been under­taken only after a well is completely clogged,thus making the clean-out effort less effective

SOURCE: Oberdo rfer and Peterson (1982).* Shou ld not be used for injection.

INJEcrION TEST REDUCTION FAcrORS TO DETERMINE

INJECTION CAPACITY

TEST FLOW RATE OF SINGLE WELL

6 / 175/28O'---~_;::_---~:;__---__;:_';.;.:;_---'

>­I-o<f

~ 4oo!!; ,o .w .a. \<J) ,

\.2 ...........

~ 6<II

"'.....E

FIGURE 6. Injection capac ity versus time for Waima­nalo, Oahu, experimental injection wells. After Oberdor­fer and Peterson (1982).

-- Well 4-3- ---- Well 4 -4-- Well 4 -5_ ••- Well 6-2- ' - '- Well 6-3

the ground surface and often are only 1-2 mdeep . Because most injection systems inHawaii operate by gravity flow, these sha llowgroundwater tables leave little room for theadditional injection head buildup that almostinevitably results from well and aquifer clog­ging effects. Fluctuations of the groundwatertab le because of tidal effects, storm waves,and groundwater recharge further add to theproblem. At some injection sites very closeto the shore , water table fluctuations of 2 mor less, when combined with clogging effects,have resulted in well overflows.

Another common cause of failure ofexistinginjection wells has been the consistent under­design of injection well capacity. Oberdorferand Peterson (1982)conclusively demonstratethat clogging effects commonly reduce initialinjection well capacities by 50% and, in somecases, by as much as 90% (Figure 6). A set ofrecommended reduction factors (Table 1) tobe applied to the injection test results wasdetermined for Hawaiian injection situationsas a way of pred icting the maintainable injec-

TAB LE 2

INIECfANT WATER Q UALITY FROM SELECfED OAHUWASTEWATERTREATMENT PLANTS, F EBRUARY 1980-DECEMBER 1981

EWA VILLA EWALANI H ALEIWA KAHUKU K ULANA MAKAUA MOKULEIA PAALAA KAI PAT'S AT WAIMANALOSURF SUGAR MILL VILLAGE VILLAGE SANDS WWTP PUNALUU WW TP

Number of samples 7 7 9 8 7 9 9 4 9 10BODs 41 52 33 6 15 27 25 10 9 13

(3~86) (19- 100) (5- 70) (1- 12) (8-20) (8-61) (6- 100) (5- 19) (3- 23) (2-32)Suspended so lids 81 81 38 29 13 44 23 9 10 10

(23-214) (\6- 239) (6-86) (2-60) (4-24) (1-260) (4- 57) (3- 15) (1- 22) (4-29)Dissolved so lids 543 606 574 430 594 351 597 509 314 296

(490-613) (512-678) (4 11- 776) (365- 550) (563- 632) (282-454) (382-795) (318-738) (267-408) (148- 388)Oil and grease 4 3 9 9 3 4 7 2 5 3

(1-7) (3- 4) (5-19) (3- 18) (2- 3) (0- 1I) (2-1 7) (1-4) (2-8) (1-5)pH 7.3 7.2 7.2 7.7 7.3 7.2 7.1 7.0 6.9 6.8

(6.9- 7.6) (6.9- 7.7) (6.7- 7.7) (6.7-8.3) (7.1- 7.7) (6.6- 7.9) (6.7-7.5) (6.4-7.4) (6.4- 7.6) (6 .0- 7.6)Alkalinity (as CaC03 ) 152 121 133 38 182 88 30 141 38 72

(122-178) (68-218) (38- 159) (7-59) (101- 2 10) (53- 127) (3-68) (9 1-198) (16-72) (28-133)Chlo rides 180 130 190 90 150 90 190 140 70 70

(140-340) (50-170) (120-260) (90-100) (80- 180) (70 - 110) (1 10-220) (120- 170) (40-100) (50-80)N0 2 + N 0 3-N 0 5 0 8 2 3 17 2 7 8

(0-2) (1-9) (0- 1) (5- 11) (0-9) (0- 11) (7-23) (\ -1 8) (0- 12)Total CI residual 0.2 0.1 0 0.7 0 0 0.2 1.1 0 I.5

(0- 0.4) (0- 0.3) 0 (0.2- 1.3) 0 0 (0- 0.4) « 0.5 - 1.5) 0 (0- 45.)

SOURCE: Oberdorfer and Peterson (1982).NOTE: Averages; ranges within parentheses; all figures are in milligram s per liter except number of samples and pH .

238 PACIFIC SCIENCE, Volume 39, April 1985

10

8

~6oxE 4<,

~ 2 I Noninject ion

5 .......

IIIIIIII

Pumpino :

Pumping ~ ;

~-~

Injectio n withUnfiltratedUnchlorinatedEffluent

Acid Treatmentand Pump ing

f"'"" ?Pumping I

~

FEB .JAN . 19 82DEC.1981

OCT.

Sho ck Chlor ination Sodium Hydroxideand Pumpi ng and Pumping

1=. i:----~-"-.;."'"

'" 20 ' Pipe Addedto Well Discharge

IIII •I. ~

Air Compr essor

SEPT.AUG.

Non in jec t ion

>- OL-"--- --:-:-:-::--- ---'-- --=-=,----'_ _ -::==--_--'-_--:-::,.,.,-_---'-_ ----:=::-__"----:-:-.,..,.--:==-----'--==__'t-

~<lU 8

~u,U 6wQ.lIl 4

FEB. MAR . APR . MAY19 82

JUNE JULY AUG.

FIG URE 7. Injection history for the Paalaa Kai, Oahu , experimental injection well. After Oberdorfer and Peterson(1982).

than earlier attempts might have been. Find­ings from our own work (Oberdorfer andPeterson 1982) and those of others indicatethat several physical and chemical techniqueshave been successful for Hawaiian injectionwells. In particular, physical flow reversalmethods, such as pumping or blowing outthe water with compressed air , and chemica lmethods, such as acid and shock chlorina­tion treatments, have proved successful inrestoring most injection capacity. Figure 7illustrates the restorative effects of various in­jection well rehabilitation methods.

To understand what the precise cloggingmechanisms are, one must examine the de­tailed geochemical and biochemical processesthat occur in the near-well environment dur­ing injection . Although injection wells arewidely used in the United States and through­out the world , few detailed investigations ofinjection well clogging have been reported.Perhaps the most comprehensive study of thissort is a compilation by Olsthoorn (1982) ofclogging problems associated with rechargewells. Other work pertinent to Hawaii's injec­tion problems has been done by the U.S. Ge­ological Survey on injection well clogging atBay Park, New York (Ehrlich, Vecchioli,

and Ehlke 1977, Ragone 1977, VecchioIi andKu 1972, Vecchioli, Ku, and Suiam 1980).The most significant conclusions from thesestudies are the following:

1. The major cause of clogging at most sitesis filtration by the porous media of sus­pended solids contained within the injec­tant.

2. A second major cause of clogging result sfrom microbial growth at the well face andwithin the aquifer pore s.

3. Chemical precipitation processes are of'les­ser significance for clogging.

4. Clogging may occasionally result from en­trapped air and gas bubbles introduced bythe injectant.

5. Most of the clogging activity occurs at orvery near the injection well aquifer bound­ary and, in many instances, a mat of fil­tration material forms directly on the wellor aquifer surface .

To determine whether these same factorsare important in clogging injection wells inHawaii, the authors conducted a series of in­jection well field experiments. In these experi­ments , which ran for almost 2 yr, secondary­treated sewage effluent was injected into sedi-

Wastewater Injection Wells-PETERSON AND OBERDORFER 239

mentary caprock receiving formations underconditions typical of those at most small pri­vate Hawaiian injection facilities. Data on in­jection head distribution and biochemicalconstituents in sediment cores and pore waterwithin about 2m of the injection wells, the zonemost likely to experience severe clogging, werecollected . These data suggest that during thefirst few days or weeks of injection, cloggingby filtration ofsuspended solids and by micro­bial growth are most important. Over the longterm , however , it appears that nitrogen gas isproduced by denitrifying bacteria in sufficientquantities to be an important contributor toclogging of pore spaces by gas binding.

These results , which are described in detailby Oberdorfer and Peterson (1982, in press)and Oberdorfer (1983), are based on experi­ments at only two injection sites and must befurther verified. If, however , nitrogen gasbinding proves to be a significant cloggingmechanism at other sites, we need to rethinksome of our ideas on clogging control andinjection well rehabilitation. To better controlclogging in the first place, perhaps more em­phasis should be placed on control of nitrogencompounds and denitrification processes; andto achieve more efficient well rehabilitation,more emphasis might be given to treatmentsthat reduce gas binding.

OUTLOOK FOR THE FUTURE

Based on injection well experience inHawaii during the past two decades, severalobservations seem appropriate. First, becauseof stringent control on the location of injec­tion wells, contamination of potable ground­water bodies by injected effluent has not been,and in the future should not be, a significantproblem. Likewise, with the possible excep­tion of a few localized areas, contamination ofshallow coastal waters should not pose a sig­nificant problem. Clogging will undoubtedlycontinue to be a major obstacle to the success­ful operation of existing and future injectionwells.

It is possible , however , to achieve consider­able improvement in injection well perfor­mance if steps are taken to eliminate existing

deficiencies. The most important of these in­volve better site selection, more realistic injec­tion capacity prediction and design, bettercontrol of injectant quality, and the use ofmore diligent well maintenance and rehabili­tation practices.

In conclusion, it is now quite clear that injec­tion wells are not the low-cost maintenance­free wastewater disposal alternative they wereonce thought to be. Furthermore, it is quitelikely that under all but the most favorable ofconditions, the useful lifetime of injectionwells is quite short, probably only a few yearsat the most, and perhaps their use should beconsidered only as an interim disposal solu­tion . Nonetheless, at favorable sites, the use ofwastewater injection wells can be moderatelysuccessful if adequate effort and money areexpended to ensure their proper operation.

LITERATURE CITED

EHRLICH, G., H . Ku, J. VECCHIOLI, andT. EHLKE. 1977. Microbial effects of re­charging the Magothy aquifer, Bay Park,New York, with tertiary-treated sewage.U.S. Geol. Surv. Prof. Pap. 751-E. u.s.Government Printing Office, Washington,D.C.

HARPAZ, Y. 1971 . Artificial groundwaterrecharge by means of wells in Israel.J. Hydraul. Div. Amer. Soc. Civil Eng.97(HYI2) :947-964.

HAWAII STATE D EPARTMENT OF HEALTH. 1984.Underground injection control, administra­tive rules. Chapter 23, title II (proposed).

HONOLULU BOARD OF WATER SUPPLY. 1982.Rules and regulations. Chapter III , section3-304 (as amended December 16, 1982).City and County of Honolulu.

OBERDORFER, J . A. 1983. Wastewater injec­tion: Near-well processes and their relation­ship to clogging. Ph.D. Dissertation. Uni­versity of Hawaii, Honolulu.

OBERDORFER, J . A., and F. L. PETERSON. 1982.Wastewater injection well problems, pro­cesses and standards. Water Resources Res.Center Techn . Rep. No . 146. University ofHawaii , Honolulu.

- - - . In press . Wastewater injection: Geo-

240

chemical and biochemical clogging pro­cesses. Ground Water.

OLSTHOORN, T. 1982. The clogging of rechargewells, main subject. KIWA Communica­tion s 72. The Netherlands Testing and Re­search Institute, Rijswijk, Netherlands.

PETERSON, F . L., J. A. WILLIAMS, and S. W.WHEATCRAFf. 1978. Wa ste injection in atwo-phase flow field: Sandbox and Hele­Shaw models study . Ground Water 16(6) :410-416.

PETTY, S., and F. L. PETERSON. 1979. Hawai­ian waste injection practices and problems.Water Resources Res . Center Techn. Rep.No . 123. University of Hawaii, Honolulu .

RAGONE, S. 1977. Geochemical effects of re­charging the Magothy aquifer, Bay Park,New York, with tertiary-treated sewage.U.S. Geol. Surv. Prof. Pap . 751-D. U .S.Government Printing Office, Washington,D .C.

PACIFIC SCIENCE, Volume 39, April 1985

REBHUN, M., and J. SCHWARTZ. 1968. Clog­ging and contamination processes in re­charge wells. Water Resources Res. 4(6):1207-1217.

VECCHIOLI, J. , and H . Ku. 1972. Preliminaryresults of injecting highly treated sewageeffluent into a deep sand aquifer at BayPark, Ne w York. U.S . Geol. Surv. Prof.Pap. 751-A. U.S. Government Printing Of­fice, Washington, D .C.

VECCHIOLI, J. , H . Ku, and D . SULAM. 1980.Hydraulic effects of recharging the Mag­othy aquifer, Bay Park, New York. U.S .Geol. Surv. Prof. Pap. 751-F. U.S . Govern­ment Printing Office, Washington, D .C.

WHEATCRAFT, S. W., and F. L. PETERSON.1979. Numerical modeling of liquid wasteinjection into a two-phase fluid system.Water Resources Res. Center Techn. Rep.No. 125. University of Hawaii, Honolulu.