This paper not to be cited without prior re~erence tp the authors International Council C.E. 1972,!E:30 ~or the Exploration o~ the Sea Fisheries Improvement Committee Re~. K (Shell~ish and Benthos C.) • BENTHIC OBSERV..TIONS ON A SEViAGE SLUDGE DmWPING GROUND By G. Topping and A. D. RiclntYre • Marine Lc.boratory Victoria Road Aberdeen Scotland BENTHIC 6BSERV1~TIONS ON A SE\1AGE SLUDGE DUl.,.IPING GROUND By G. Topping und A. D. Mclntyre f.larine I.aboratory, Aberdeen, Scotland SUliIliARY Recent observations including chemical analysis o~ sediments, and grab and TV studies of the macrofauna, are presented for asewage sludge dumping ground in the Firth o~ Clyde, und comparisons are made with clean ground elsewhere in the Firth. L distinct gradient is demonstrated from a balanced diverse benthic commuluty in the clean area through an enhanced ~a~ in the transition area on the edge of the dump ground and culminating at the centre o~ the ground in a fauna o~ very low diversity composed almost entirely of two species of worms, but present in very large numbers. A long period of substantial dumping has had a marked effect but there was no question of azoic conditions having developed • o~ and the effected area is comparatively small. A build-up heav,y metals in the sediment has however been demonstrated. Hfi'RODUCTION Dumping at sea constitutes an economic method of the disposal of sewage sludge, and a number of major industrial cities are at present employing this technique ~or a considerable proportion of their sewage waste. Table 1 indicates the annual quantities of such discharge by four British and one American city. Although disposal at sea has been employed for some time (Glasgow and London began in the early part of this century), it is only recently that attempts have been made to assess the effects of such discharges (Shelton 1970; Macl::ay and Topping 1970;· Pearce 1970; Mackay et al 1972) • This report presents ti1e preliminary results of surveys carried out • by the Earine Laboratory, Aberdeen in the.Firth of Clyde in Scotland in an area 3 miles south of Garrochhead, Bute (see Figure 1) to assess the physical, biological and chemical effects of sewage sludge on fauna and sediments. Apart from the period 1940-45, this area has been used for dumping by the city of Glasgow since 1904, and in recent years the load has included contributions from other local authorities. The material dumped is a combination of settled sewage and solids from activated sludge taclcs together with small quantities of industrial waste. It is collected at or brought to holding tanks sited at Shieldhall and Dalmuir, in the city of Glasgow, from 3 main sewage works (Shieldhall, Dalmuir and Dalmarnock). From there i t is loaded into specially designed hopper barges for ultimate transport to the dumping grounds at Garrochhead. The two vessels .employed for this operation have a capacity of 1500 tons and make one trip per day throughout the year. Arecent analysis of the dumped material is given in T~ble 2, where range~ of values for volatile material, nitrogen, phosphorous, fat and grease.and heavy metals are listed·~or a composite sample from the main three sewage works. Recent analysis of the sludge by the staf~ of t,he 1 Dalmarnock Sewage Works for its mercury content has revealed a range of values from 0.104 to 0.173 ppm wet weight, and ana~sis for PCB's has shown there has been a steady decline from about 1 ppm wet weight in 1971 (1 ton in total) to less than 0.5 ppm wet weight in 1972. METHODS Field Worlc ü thin the knorm area of the dumping ground a network of 18 stations was sampled (Figure 1) to establish the approxiuate centre of 2 the deposits, using a 1/10 m grab. The main parameters used immediately at sea to assist in delimiting the dump centre were sediment texture and associated odo~r. At.each station samples of surface sediment, which were known to have no contact vdth the metal parts of the ßTab, were collected and transferred to plastic containers far chemical analysis. The larger benthic organisms were removed from t:'lese samples but no further attempt was made to sort them. On a li:nited number of occasions, samples of bottom water were collected for analysis of dissolved oxygen. At selected stations the complete grab haul was sieved throu~1 a 0.5 mm screen and • the residue preserved in ~~ formalin fnr identification ~nd enumeration of the l'l.lacrofauna. At additional stations, mainly off the north east coast of Arran, and off the \yrshire coast between Ardrossan and Troon, further samples were collected for comparison vdth those of the dump area. Finally direct examination of the sea bed was made on several occasions using underwater television equipment at various locations in the Clyde. In particular a traverse was examined extending from sludge­ free grounds off the Arran coast to the centre of the dumping area. Notes were made at sea on the bottom conditions and on the type of fauna observed, and the videotape was later studied in detail at the labora~ory when quantitative counts could be made of features and fauna. Analytical Technigues Samples of sediment were dried to constant weight at 100°C. Sub-samples of this [;round material were then a ssayed for . volatile material by d~ ashing at 5000 C and for a henvy metal content by wet ashing with nitric/perchlaric acids followed by atomic absorption S' ectroscopy. Bacl~ground corrections for matrix effects were made using a hydroben lamp continuum s~urce. • RESU'LTS ~lved Oxyge!1 This was measured at the dump area on two surveys when the percentage saturation of the bottom water was 94.5 and 97.5. These values were at least as high as those recorded from the upper water layers of the dump ground, and frbm other locations on the ~lyde. The situation is thus similar to that described by Shelton (1970) for dumping grounds in the outer Thames estua~, but different from that of New York described by Pearee (1970) who found depressed levels of dissolved oxygen over the dump grounds. Sediments At depths off Arran comparable to the dumping ground, the sediments were odourless and the surface layers were of fine smooth grey mud. On the dumping ground there was a marked change and the centre of 2 the ~~e~ could easi~ be delimited by the appear~nce and textura ot the aedimen~s .whioh ware black and ~brous and had a strong smell. Analysis of ~he volatile oontent a~d the co~centrations of copper, zinc, lead and oadmium are ßiv.en in Table 3 for a grid of stations covering the dumping ground.. It is ciear that stations 3, 6. and 13 ga,?e higher . va1ues tor all measurements. Comparison of ,these data with ooncentra:~­ ions of the same substances at 30 other stations outside the dump area gave the ranges and m.ean values shown in Table 4. From the sediment studies, the oent~ of the dumping ground appears to be at stations 1, ·2, 3, 6, 7, 8 and 13. These findings are in broad.agreement with those of r,iackay and Topping (1970) and Hackay et a1 (1972) tor this area. Grab Surveys Data on the ua crofauna are given .in Table 5 from stationa in the centre of the dumping ground; on the edgl;l of the ground; and also from stations on Qlean ground at the same depth .to the south west, off the north east ooast of ~'~rran. The Arran ground can be considered as a "control" area, shoV(.i.ng a fauna typical of clean muds in the. Clyde at about 90 metres depth. In this area there is a diverse fauna mainly of errant and sedentar,y polyc~etes, moll~sos, orustaoeans and eohinoderms. At the centre of the dumping ground there was a marked differenoe in the fauna; no mollusos or crustaceans ware found and no echinoderms except • for a fevv juvenile ophiuroids which had probably .drifted on ~o the area. as larvae. Annelid worms made up 'almost all the fauna, and even within this group the diversity w~s drastioally reduced. Spionids were complete~ absent.a~d olig9c~aet~~.and capitellids, opportunistio taxa known to thrive on organically polluted areas, ware present in very., high numbers, along with several speoies o~ cirratulids. On the edge of the dumping ground 1ess than two miles from the centre stations, where the deposit was clean and without smell the diversity was greater, with both molluscs and crustaceans present but spionid polychaetes were still absent and a nonnal echinoderm fauna was not found. TV Surveys. The impression gained from the grab surveys was 8ubstantiated by detai1ed examination of the TV videotape talcen while drif'ting fram the oontral aree off Arran to the centre of the dumping ground. In the oontrol a.rea many tracks of surface living animals were always present end gastropods were frequently observed. Bup-o~s of lif:lphrops norvetzious ware seen in densities of about 12 per metre and holes of .various aize indioating the lresenoe of ~rti.cel burrows, p~bably of crustaoeans or molluscs, were counted in numbers of up to 90 m on many frames. These • are all indications of the presence of anima1s whioh would not ea~i1y be sampled by the grab. Also present on the surfaoe were numerous eohinoids, in particular speoies of Ityssopsis, could be reoo8niaed. As the dumping ground was approaohed, first the echinoderms declined in numbers, then the burrows of OI'Ustaoeans and molluscs began to disappear until towards the centre of the ground ell evidence of these a~la was lacking and the alterations in the texture of' the sediment, wer,e obvious even on the TV soreen.