Lake Treh6rningen restoration project. Changes in water quality after sediment dredging

Sven-Olof RydingNational Swedish Environment Protection Board, Research Department, Water Quality Laboratory,Box 8043, S-750 08 Uppsala, Sweden

Keywords: lake restoration, sediment dredging, nitrogen, phosphorus, algal assays

Abstract

An increased load of domestic wastewater to Lake Trehorningen induced oxygen-poor water conditionsand the development of a reduced sulphide-rich sediment layer. Severely polluted, the lake did not recover,even after advanced wastewater treatment and sewage diversion. Restoration measures with suction dredgingand macrophyte elimination were applied in 1975 and 1976. The loose topmost sediment was pumped into anembanked and overgrown bay which was used as a settling pond. The activities also included a restoration ofthe shorelines. This project is the largest restoration programme carried out in Sweden on a single lake,corresponding to a cost of about US $2 000 000.

The restoration of Lake Trehorningen was followed by a highly intensive research programme whichincluded water chemistry and algal assays. The concentrations of phosphate and total phosphorus decreasedby 73 and 50% respectively, as summer average values, two years after the restoration. However, theconcentrations of phosphorus are still too high to permit this element to act as a prime algal growth-limitingnutrient. The algal biomass has also remained at the same magnitude as before the restoration. Nitrate-Nconcentrations showed a tenfold increase, based on average values for the summer period. However, based onthe results of the algal assays, a rapid and marked response was obvious, with a drastic decline in the algalgrowth potential.

In addition, the water quality of the tributaries was frequently of an objectionable character (0.1-0.2 g Pm 3 ). The nutrient loading from these sources exceeds the critical level for the lake, and measures have nowbeen carried out to treat all the inflowing waters for the removal of phosphorus.

Introduction

Lake degradation now constitutes a major envi-ronmental problem, and has become the subject ofmuch public and scientific concern. Although lakequality has been impaired by the addition of toxicsubstances, temperature changes and radioactivity,the most prevalent problems occur as a result ofeutrophication and sedimentation. The effects inthe water bodies can be synergistic, reflecting thetransport of nutrients to the lakes and the produc-tion of organic matter within the lakes, throughnutrient utilization by aquatic organisms. These

processes cause or aggravate the familiar lake prob-lems of nuisance growths of planktonic and att-ached plants, offensive odours, turbidity and sedi-ment in-filling, and result in less usable recreationalwater surface, changing fisheries and fish kills.Many situations call for water quality maintenanceor protection efforts. Where it is too late for such anapproach, lake rehabilitation may be warranted;this is receiving increased consideration and re-search evaluation, as an option for the managementof natural resources.

Approaches to lake restoration fall into twocategories, methods to limit fertility and/or sedi-

Hydrobiologia 92, 549 558 (1982). 0018-8158/82/0922-0549/$02.00.© Dr W. Junk Publishers, The Hague. Printed in The Netherlands.

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mentation in lakes, and procedures to manage theconsequences of accelerated lake aging (for surveysof lake rehabilitation techniques and experiencessee Bjork 1972; Dunst et al. 1974; Ryding 1981).Removal of nutrient-rich sediments (dredging),deposited in eutrophicated and over-exploited re-ceiving waters, falls into the second category andmust be considered as one of the most radicalmethods of restoring damaged lakes. Lake Trehor-ningen, situated in central Sweden, in the vicinity ofStockholm, is a good example of the large group ofpolluted lakes which have become deterioratedwithin urbanized areas. The pollution from theHuddinge municipality became more and more se-vere and, in the 1950s, the lake became thoroughlydegraded. Despite advanced wastewater treatment,and the later diversion of sewage, the lake did notrecover. As the lake has a high recreational valuethe authorities decided to restore it. In 1975 and

1976 the topmost nutrient-rich layer of the sedi-ments was removed by suction dredging. The resto-ration of Lake Trehorningen was followed by ahighly intensive limnological research programme.This paper summarizes the first evaluation of theresearch data and it describes the first signs of re-covery of the lake, after the remedial efforts.

Background data and methods

Lake TrehSrningen is part of the watercourse ofthe River Tyresan. The lake has a total area of0.65 km 2 and is fairly shallow, with a maximumdepth of about 3.5 m. The lake basin is divided intotwo sub-basins (Fig. 1), the western basin beingsomewhat deeper than the eastern part. The watersentering the lake are greatly influenced by humanactivities. The drainage areas and the differentland-use patterns for the three rivers in the catch-ment area are presented in Table 1. Only 22% of thetotal area of the drainage basin consists of forestsand parks, only 1% of the area is covered by lakes,and over 75% of the basin is urbanized and settled.

The restoration of Lake Trehorningen includedseveral phases:1) impounding an overgrown bay of the lake (Lan-

naviken, see Fig. 1), to serve as a settling pond;2) construction of pipes, for by-passing urban

stormwater through the Lannaviken settlingpond;

3) elimination of dense macrophyte vegetation,thereby restoring the shoreline;

4) dredging the topmost half meter of the sediment(average depth);

Table 1. Areas and land-use patterns for the three drainage basins of Lake Trehorningen.

River River RiverGommarbacken Solfagradiket Oullarangen Total

Forests and parkskm 2 2.00 0.10 1.00 3.10% 26 2 100 22Lakeskm 2 0.20 - - 0.20% 3 1Buildingskm 2 1.00 0.60 - 1.60% 13 11 11Houseskm2 4.20 4.80 - 9.00% 53 87 - 63Industrykm 2 0.40 - - 0.40% 5 3Totalkm2 7.80 5.50 1.00 14.30% 55 38 7 100

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5) treating the run-off water from the Linnavikensettling pond, a mixture of lake and interstitialwater, with aluminium sulphate in a plant forprecipitation of phosphorus and suspended mat-ter.The sediment was dredged to a depth of 0.2-1.0 m

depending upon the thickness of the nutrient-richupper layer. Altogether about 320 000 m3 sediment(gyttja) was removed. This material and the macro-phytes were deposited into the Lannaviken settlingpond, which had an area of 150 000 m2. Due to thelimited volume of the settling pond (290 000 m3),the sediment dredging had to be carried out mainlyduring two summer periods, 1975 (the western bas-in) and 1976 (the eastern basin). The effluent waterfrom the settling pond was pumped to a plant forchemical treatment at a rate of 200 m3 h-1 . Duringthe dredging phases no water was allowed to leavethe lake.

One result of the sediment dredging was a deep-ening of the lake to a mean depth of more than1.5 m. Following the removal of most of the mac-rophytes surrounding the lake, the surface area wasexpected to remain constant, in the future. Someseparate stands of macrophytes, and smaller bays,were left untouched for waterfowl reservations.

The limnological investigations, to study theprogress of restoration after the removal of sedi-ments, were mainly based on water chemistry anal-yses in the tributaries, the lake and the outlet.Sampling was frequently carried out on the twosub-basins. Surface water samples were taken oncea week during the growing period. From a well-de-fined sampling area several samples were taken andcarefully mixed. Water samples from the inlet andoutlet were taken once a week, all year round. Sim-ilar observations of the discharge were made, topermit mass balance studies. The water chemistry

analyses included several standard water qualityparameters, giving fractions and total amounts ofnitrogen and phosphorus (NH4 -N, NO 3-N, tot-N,PO4 -P and tot-P), organic matter (COD) and phy-toplankton biomass (chlorophyll a). The algalgrowth potential (AGP) was studied with algal as-says using the test algae Selenastrum capricornu-tum, Printz, Nyg (= Monoraphidium capricornu-tum, Printz, Nyg,) by Dr. A. Claesson (see Claesson& Forsberg 1978). The analytical procedures usedhave been described by Ryding (1978).

Results

Inflowing waters

The catchment areas of River G6mmarbackenand River Solfagradiket cover, together, over 90%of the total drainage basin of Lake Trehoirningen.The water quality in these two tributaries was fre-quently of an objectionable character. The RiverGommarbacken had the highest concentrationvalues of total phosphorus, 0.180 g P m-3 (Table 2)

Table 2. Water quality in River Gommarbacken and RiverSolfagradiket. Average values and ranges (based on monthlyaverage values) for the period 1975-1977, g m-3.

River RiverGommarbicken Solfagradiket

Total nitrogen 1.870 2.5601.240-3.430 1.300-4.790

Total phosphorus 0.180 0.1450.038-0.625 0.053-0.399

N/P ratio 10.4 17.61.9-90.3 3.9-90.4

Organic matter 24 4916-43 23-167

Table 3. Lake Trehirningen, 1975-1978. Hydraulic load and supply of nutrients and organic matter from the different drainage basins.Average annual values, 106 m3, kg and kg · 103 respectively.

River River RiverGommarbacken Solfagradiket Gullarangen Total

Discharge 2.080 1.470 0.270 3.813% 55 38 7Total nitrogen 3 906 4 112 353 8 371% 47 49 4Total phosphorus 302 159 47 508% 60 31 9Organic matter 48.3 62.2 3.3 113.5% 43 55 2

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as an average for the observation period 1975-1977.The water in the River Solfagradiket was compara-tively rich in nitrogen, resulting in a high N/ P ratio.The pollution by organic substances, to the RiverSolfagradiket, seemed to be far more pronounced,resulting in more than twice the concentration oforganic matter than occurred in the River Gom-marbacken.

The loading of nutrients and organic matter inLake Trehorningen are presented in Table 3. Theloading figures from the River Gullarangen havebeen estimated by areal proportioning. The differ-ent characteristics of the water quality of theserivers were also reflected in their loadings to thelake. Consequently, the main part of the phospho-rus inflow originated from the River G6mmar-backen. The River Solfagradiket was the dominantsource for the supply of nitrogen and organicmatter. The River Gullarangen contributed least to

the total transport of elements into Lake Trehor-ningen.

The total transport of nutrients and organic mat-ter, on a monthly basis, fluctuated widely duringthe observation period, 1975-1978 (Fig. 2). Thegeneral pattern of variation was similar for nitro-gen, phosphorus and organic matter. The varia-tions appeared to be most directly related to waterflow, which usually has the greatest impact on theloading regime. The deviations in the data are most-ly expressions of the differences in water qualitywhich, as pointed out above, were considerable (cf.the ranges in Table 2).

Great differences in the hydraulic regime werenoted from year to year (Table 4). This was espe-cially the case in 1977, during which there was adecrease in the theoretical hydraulic residence timeby a factor of 2-3, to 0.16 years. The rather lowratio between the area of the total drainage basin

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Table 4. Hydraulic residence times (years) and the areal annualload of total nitrogen and total phosphorus (g m 2) on LakeTreh6rningen 1975-1978.

1975 1976 1977 1978

Hydraulic residence time 0.47 0.43 0.16 0.37Total nitrogen 7.3 7.7 25.6 10.8Total phosphorus 0.70 0.74 0.88 0.81N/P ratio 10.5 10.3 29.3 13.4

and the lake surface area, 22:1, results in a fairlylong hydraulic residence time, despite the small lakevolume. The average hydraulic residence time, forthe years 1975, 1976 and 1978, was about 5 months.The annual supply of phosphorus to Lake Treh6r-ningen was very constant. The annual load of nitro-

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Lake water

During the first year of dredging (in the westernbasin) lake water sampling was carried out only inthe eastern basin, thereby serving as referencevalues for the water quality, before restoration.Therefore a fair judgement of the recovery of thelake, in terms of changes in surface water quality,ought to be related to conditions observed in theeastern basin. The short-term variations on a week-ly basis, for total nitrogen, total phosphorus andchlorophyll a are illustrated in Fig. 3. Water quality

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showed rapid and great variations. A lowering ofthe concentrations of these three parameters maybe observed during the first year (after the dredgingin the western basin). Somewhat lower phosphorus

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Fig. 4. Changes in water quality (algal growth potential) following the restoration of Lake Trehorningen. Weekly average values. Surfacewater (0-2 m.). Data from A. Claesson (unpublished).

Table 5. Lake Trehorningen, 1975 1978. Surface water quality (0-2 m). Average values, June-September.

Transpa- pH Susp. NH 4-N NO3-N Tot-N PO,-P Tot-P Tot-N/ COD Conduc- Chloro-rency matter Tot-P tivity phyll a(m) (g m3) (g m3 ) (gm 3 ) (g m3 ) (g m3 ) (g m3) (g m-3 ) (mS m) (mg m3 )

Western 1976 0.3 8.4 47.4 0.037 0.001 1.790 0.042 0.205 8.7:1 41 38.9 73basin 1977 0.5 8.9 47.1 0.050 0.006 2.340 0.035 0.136 17.2:1 33 28.5 56

1978 0.4 8.5 45.0 0.372 0.085 2.970 0.058 0.229 13.0:1 42 30.2 79

Eastern 1975 0.5 8.9 29.2 0.233 0.006 2.860 0.228 0.436 6.5:1 61 32.3 75basin 1976 0.4 8.9 40.1 0.028 0.002 2.180 0.079 0.268 8.1:1 43 37.9 88

1977 0.6 9.1 25.3 0.030 0.006 2.180 0.044 0.161 13.5:1 35 32.2 611978 0.4 8.7 47.4 0.323 0.056 3.220 0.063 0.226 14.2:1 48 28.4 76

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Changes in water quality, after the restoration,may be more easily illustrated by presenting thesummer average values for separate years (Table 5).The concentrations of phosphate and total phos-phorus decreased in the eastern basin by 73 and50'0, respectively, between 1975 and 1978. The con-tent of organic matter decreased by 22%. The N/Pratio showed a continuous increase from 6.5:1 to14.2:1. A tenfold rise in the content of nitrate wasalso noted in the eastern basin. In the western basinthere were no changes in the concentrations ofphosphorus and organic matter, but an increase inall nitrogen fractions was observed. No changes inthe chlorophyll values, or transparency, were notedin either of the basins.

A rapid and marked response, following the res-toration, was obvious from the results of the algalassays (Fig. 4). The high values for algal growthpotential (in terms of number of cells and cell vol-ume) in the eastern basin, especially at the end ofthe 1975 summer period, did not occur during 1976and 1977. The values declined by several orders ofmagnitude and remained at the same level over theentire lake basin in these years.

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Discussion

There are many difficulties associated with re-medial efforts. Lakes are complicated ecosystemsand our ability to predict the response of lakes, tovarious treatments, is still limited. Moreover, eachlake has its own 'unique personality', which frus-trates attempts to transfer results from one lake toanother even though they appear to have the sameproblems. A certain amount of pre-treatment in-formation is required, in order to be able to formu-late a well-founded remedial programme, e.g., onnutrient fluxes to and within the water body. Theeconomics of a particular lake restoration effortcan also affect the choice of treatment alternatives,and there are also time constraints associated withrenewal programmes. The public wants prompt ac-tion, and immediate results. This is seldom possi-ble. Natural variations may mask initial changes,brought about by lake treatment, so that manyyears may be required to demonstrate that realchanges have taken place.

In the early 1970s, prior to the restoration ofLake Treh6rningen, a comprehensive pre-investi-gation assessment was carried out, to obtain thenecessary information for the preparation of anappropriate restoration plan (Orrje & Co 1973).Historical water quality data revealed that LakeTreh6rningen had already shown the typical symp-toms of an overfed lake, early in the 1960s. Theconcentrations of phosphorus were extremely high,with a value of 1.7 g P m-3, as an average. Sedimentinvestigations showed that the bottom was coveredby a 10-20 cm layer of reduced sulphide-rich sedi-ment. This indicated that there was an excessivenutrient and organic load and that high microbialactivity was associated with oxygen-poor waterconditions. High concentrations of several metals(Ni, Cr and Cu) were found in the sediment. Thecontent of phosphorus varied between 1.2 and 6.7 gkg-1 DW. A 4 m sediment core from the lake indi-cated that most of the vertical sediment profileconsisted of a homogeneous 'plankton-detritus-gyttja', deposited during the 3 300 years since thelake was isolated from the Baltic. The natural sedi-ment accumulation was estimated to about I mmyr-l. The increasing artificial load to the lake, fromthe beginning of this century, had increased thisvalue to about 5 mm yr-l . Despite the lowering ofthe nutrient and organic load by advanced waste-

557

water treatment, and later a total diversion of sew-age, the water quality showed no signs of improve-ment. The main reason for this was shown to be thecontinuous circulation of nutrients between sedi-ments and water (internal loading) which wouldcause poor water quality for many years to come.Dredging the topmost nutrient-rich sediment wasconsidered to be the best technique to apply in thelake, to overcome this problem.

One of the most well-known examples of a eu-trophicated lake, which has undergone sedimentdredging, is Lake Trummen, in south-central Swe-den. Lake Trummen was restored in 1970 and 1971by a technique similar to that described above, forLake Treh6rningen. Lake Trummen respondedpositively and rapidly to the restoration efforts,with rapidly decreasing nitrogen and phosphorusvalues (Bengtsson et al. 1975), as well as loweredphytoplankton production (Cronberg et al. 1975).The delayed recovery observed in Lake Treh6rnin-gen may be somewhat confusing, if based on asimple comparison with the positive response re-ported from Lake Trummen. The better conditionsobserved in Lake Treht3rningen, in 1977, may havetheir explanation in the higher flushing rate of thatlake during that year. A closer look at some of theresults, however, reveal some additional differen-ces. Firstly, the load to Lake Trehirningen, from itssurroundings, still contributes a large amount ofnutrients and organic matter. According to the ex-ternal loading criteria (Vollenweider 1968) the arealnitrogen and phosphorus loads far exceed the criti-cal level. Taking the hydrological conditions intoaccount, in the later modifications of the loadingcriteria concept (Vollenweider 1975), this situationis repeated, but it is not as pronounced. Althoughthese model approaches, by Vollenweider, wereformulated for deeper stratified lakes, they can beused to give an indication of the loading conditionsin smaller shallow lakes. The emerging awarenessof the prevailing high nutrient load to Lake Tre-h6rningen forced supplementary actions to be car-ried out in the restoration plan. Nowadays, all in-flowing waters from the River Gommarbacken andthe River Solfagradiket are temporarily stored,within the lake and in separate flow-equalizing bas-ins, for phosphorus removal by chemical treatment(Dunkers & S6derlund 1978).

Secondly, the nutrient release from the sedimentsseems to influence water quality, to a great extent.

The internal loading in shallow lakes, generally, hasan annual regime, with its maximum during thesummer (Ryding & Forsberg 1977) which alsocoincides with the period of minimum externalload. By combining the external and internal load-ings of phosphorus on shallow lakes, in Fig. 5, itcan be shown that the problems of over-fertilizationfrom internal sources must be curbed, in order to beable to control and achieve good water quality dur-ing the summer period. A large part of the externalload to a lake, at the beginning and at the end-of ayear, may be of little concern since, in lakes with ahigh flushing rate, these nutrients may pass throughor reach the lake at times when they are unable toinfluence the summer algal production (Ryding &Forsberg 1980a). The peak values of phosphorus inLakeTrehorningen(cf. Fig. 3) are an indication thatthe role of the sediments, in recycling nutrients, hasnot yet been minimized. Laboratory investigationson the release rates of phosphorus from sedimentstaken from Lake Trehorningen gave the followingresults, under different conditions (Orrje & Co1973):

Undredged - aerobic conditions 12 mg P m-2 day'Dredged - aerobic conditions 6 mg P m 2 . day Undredged - anaerobic conditions 15 mg P m2 . day-'Dredged - anaerobic conditions 7 mg P m-2 · day'

These values may be compared with the maximumrelease rates, of 44, 47 and 49 mg P m 2 day',observed from field studies in three Swedish hyper-trophic shallow lakes: Lakes S6dra Bergundasjoin,Glaningen and Finjasjin (Ryding & Forsberg1980b). The decrease in the phosphorus releaserates of sediments from Lake TrehiSrningen, esti-mated from the laboratory studies, seems to befairly low.

In both Lake Trummen and Lake Treh6rningenthe decline in algal growth potential showed posi-tive response after the restoration (Fig. 4 in Bengts-son et al. 1975; see also Claesson & Ryding 1980).This indicates a decrease of the 'nutrient pool'.From chemical analyses, the decrease was notedonly for phosphate, while the content of inorganicnitrogen remained at the same concentration level(Table 5 in Bengtsson et al. 1975). In Lake Treh6r-ningen, however, the phosphorus concentrationswere still too high (>0.100 g P m-3 ) to permit thiselement to act as a prime algal growth-limitingnutrient. Enrichment experiments with the algal

558

assays also verified the role of nitrogen, as an im-portant regulator of phytoplankton production inLake Trehdrningen (A. Claesson, unpubl.).

It is too early to assess the final outcome of therestoration of Lake Trehorningen. This paper hasattempted to evaluate the changes in water qualitybased on a three-year observation period. A morecomprehensive evaluation of the information in-cluding data from 1979 to 1980, and a mass-balancestudy of phosphorus, is in preparation. Despite theresearch and refinements that are still needed, sig-nificant advances in lake rehabilitation have beenmade. Hopefully, the experiences obtained, fromthe large-scale project on sediment dredging inLake Trehorningen, may contribute to the estab-lishment of sound techniques for highly effectiverestoration procedures.

References

Bengtsson, L., Fleischer, S., Lindmark, G. & Ripl, W., 1975.Lake Trummen restoration project. I. Water and sedimentchemistry. Verh. Int. Verein. Limnol. 19: 1080-1087.

Bjork, S., 1972. Swedish lake restoration program gets results.Ambio 1: 153-165.

Claesson, A. & Forsberg, A., 1978. Algal assay procedure withone or five species. Minitest. Mitt. Int. Verein. Limnol. 21:21-30.

Claesson, A. & Ryding, S.-O., 1980. Characterization of therecovery processes in hypertrophic lakes in terms of actual(lake water)- and potential (algal assay) chlorophyll. In:Barica, J. & Mur, L. R. (Eds.) Hypertrophic Ecosystems, pp.281-289. Developments in Hydrobiology, Vol. 2. Junk, TheHague.

Cronberg, G., Gelin, C. & Larsson, K., 1975. Lake Trummenrestoration project. 1I. Bacteria, phytoplankton and phyto-plankton productivity. Verh. Int. Verein. Limnol. 19:1088-1096.

Dunkers, K. & Soderlund, H., 1978. Sjbrestaurering och dagvat-tenbehandling -nya metoder - nya aspekter. Vag- och vat-tenbyggaren 10: 31-32.

Dunst, R. C., Born, S. M., Uttormark, P. D., Smith, S. A., Ni-chols, S. A., Peterson, J. O., Knauer, D. R., Serns, S. L.,Winter, D. R. & Wirth, T. L., 1974. Lake rehabilitationtechniques and experiences. Dept. Nat. Resour., Tech. Bull.No 75, Madison, Wisconsin. 179 pp.

Orrje & Co, Skandiaconsult, 1973. Teknisk beskrivning verrestaureringen av sjon Trehorningen. Orrje & Co, Skandia-consult, 52.0011-08, 1973.01.18. 21 pp.

Ryding, S.-O., 1978. Recovery of polluted lakes. Loading, waterquality and responses to nutrient reduction. Acta Univ. Up-sal., Abstracts of Uppsala dissertations, No. 459, 44 pp.

Ryding, S.-O., 1980. Monitoring of inland waters. OECD eu-trophication programme. The Nordic project. ScandinavianCouncil for Applied Research, Secretariat of EnvironmentalSciences, Publ. 1980: 2. 207 pp.

Ryding, S.-O., 1981. Reversibility of man-induced eutrophica-tion. Experiences of a lake recovery study in Sweden. Int.Revue ges. Hydrobiol. 66: 449-503.

Ryding, S.-O. & Forsberg, C., 1977. Sediments as a nutrientsource in shallow, polluted lakes. In: Golterman, H. L. (Ed.)Interactions between Sediments and Fresh Water, pp.227-234. Junk, The Hague.

Ryding, S.-O. & Forsberg, C., 1980a. Short-term load-responserelationships in shallow, polluted lakes. In: Barica, J. & Mur,L. R., (Eds.) Hypertrophic Ecosystems, pp. 95-103. Devel-opments in Hydrobiology, Vol. 2. Junk, The Hague.

Ryding, S.-O. & Forsberg, C., 1980b. Hydrologi, amnestrans-port och vattenkvalitet i Finjasj6n 1976-1978. HassleholmsKommun, Gatukontoret, April 1980. 41 pp.

Vollenweider, R. A., 1968. Scientific fundamentals of the eutro-phication of lakes and flowing waters with special referenceto nitrogen and phosphorus as factors in eutrophication.OECD DAS/CSI 68.27, Paris. 254 pp.

Vollenweider, R. A., 1975. Input-output models with specialreference to the phosphorus loading concept in limnology.Schweiz. Z. Hydrol. 37: 53-84.