during the initial stages of operation of the metal leaching step of the process according to the present invention , the leachate entering bioprecipitation will possess a neutral ph . therefore , a portion of this liquor can be used to dissolve the gaseous hydrogen sulfide effluent produced from bioprecipitation . the two aqueous hydrogen sulfide streams derived from bioprecipitation may be employed separately , or preferably combined , and oxidized within an enclosed bioreactor . the bioreactor may comprise a known system containing a consortium of naturally occurring sulfide oxidizing organisms . examples of micro - organisms known to oxidize soluble sulfides include : thiobacillus thioparus , t . neapolitanus , t . denitrificans and thiomicrospira species . two routes are possible for sulfide oxidation : ( b ) oxidation to elemental sulfur , which can if appropriate then introduced into the contaminated soil to produce sulfuric acid . oxidation to elemental sulfur requires an oxygen limited environment , but possesses the advantage of providing a sulfide - free , ph neutral liquor that can be used to dissolve effluent hydrogen sulfide gas from bioprecipitation . the sulfuric acid liquor produced by direct oxidation is more versatile for use in subsequently contacting the contaminated soil . as noted above , the process of the present invention includes one or more steps for the removal of organic contaminants from the said contaminated medium and this may be by a remediation process deployed in a similar manner to that used for the metal removal process . in general , different micro - organisms are known to degrade different species of organic compound , and appropriate microbial consortia may be selected according to the type ( s ) of compounds to be degraded but will generally be present within the contaminated material . the contaminated material is preferably analyzed prior to treatment to ensure an appropriate consortium is already present or added to be present . examples of degradative strategies which may be selected are given as follows . as a result of the interest in and research performed in the prior art to investigate the microbiological degradation of organic contaminants , several known key strategies have emerged . the strategies employed are greatly influenced by oxygen which may either function as a preferred electron acceptor , or may enzymatically be incorporated into the molecule . alkanes -- may be degraded aerobically by micro - organisms belonging to several genera including : pseudomonas , nocardia , mycobacteria , and flavobacteria . the degradation of such compounds initially involves the introduction of oxygen into the molecule by a monooxygenase enzyme . subsequent conversion of the resulting fatty acids to aldehydes and the carboxylic acids enables further oxidation through the beta oxidation pathway ( gottschalk , 1986 ). alkenes and alkynes -- can be degraded either aerobically or anaerobically . aerobic degradation occurs by a mechanism similar to that for alkanes . however , the more reactive nature of the double and triple bonds also permits initial degradation of the molecule under anaerobic conditions by either hydration , or epoxidation reactions . subsequent oxidation then proceeds via beta oxidation . halogenated aliphatic compounds -- are susceptible to both aerobic and anaerobic degradation . generally , however , more highly halogenated compounds are more susceptible to anaerobic degradation . cyclic and aromatic compounds -- are once again susceptible to both aerobic and anaerobic degradation . under aerobic conditions the initial attack involves the insertion of a series of oxygen atoms into the molecule by oxygenase enzymes . subsequent degradation occurs by either ortho or meta fission involving a further dioxygenase enzyme to achieve ring breakage . halogenated compounds are degraded by a similar mechanism . the microorganisms involved in such degradations include : species of alcaligenes , pseudomonas and corynebacteria which are able to degrade polychlorinated biphenyls ( unterman et al 1988 ) and flavobacteria species which are able to degrade pentachlorophenol ( frick et al 1988 ). under anaerobic conditions substituted aromatic compounds are reduced to cyclohexanone . ring cleavage is then achieved by hydration . aromatic compounds with more than one chlorine atom are reductively dehalogenated prior to conversion to cyclohexanone . halogenated compounds -- particularly those possessing more than one functional halogen group , are also subject to reductive dehalogenation . this involves the compounds acting as electron acceptors and results in chlorine atoms being eliminated from the molecule to be replaced by hydrogen atoms . highly halogenated compounds e . g . hexachloroethane are strongly oxidized and possess greater electron affinities than molecular oxygen . as successive rounds of dehalogenation occur and electron affinities fall , the use of alternative electron acceptors such as oxygen and nitrate becomes probable , thus governing the conditions and groups of organisms that are able to effect degradation . examples of organisms involved in reductive dechlorination include : pseudomonas , alcaligenes and clostridia sp . in contrast to metal removal , the mechanisms employed to degrade organic soil or land contaminants will be very site specific , as these will need to be tailored to the particular contaminants present within a site . however , some generalizations can be made : 1 . the degradative process will be optimized to reduce and / or eliminate a range of organic compounds particularly vocs ( volatile organic compounds ) and pahs under aerobic or anaerobic conditions . these compounds will be mineralized to co 2 and h 2 o . 2 . with the exception of a source of sulfate and possibly an anaerobic environment , the nutrients required to promote the growth of degradative organisms will be the same as during metal bioleaching and will be required at similar concentrations . 3 . near neutral ph conditions will be required to maximize the numbers and types of degradative organisms which can be grown . 4 . similar types of soil treatment equipment will be required for the degradation of organics , as are required during metals removal . in some instances organic contaminants present together with metallic contaminants are desirably treated before significant acidification , or metal mobilization within the contaminated material , as this could have a deleterious effect on the micro - organisms required in the organics degradation step . therefore , organics may initially be degraded during operation of the process according to the present invention prior to metal leaching . further organics degradation may occur during metal leaching . depending on the degradative requirement of the organic contaminants , the system may be operated aerobically , anaerobically , or a combination of the two . however , anaerobic operation would delay acidification of the contaminated material . additionally , if large amounts of organic contaminants are present it may be necessary to delay the acidification process until sufficient organic degradation has occurred . for instance , for a halogenated compound such as trichloroethylene , anaerobic conditions may be maintained to allow reductive dechlorination to vinyl chloride which may subsequently be mineralized under anaerobic conditions . following degradation of the majority of the organics , the metal removal system may then be started . additionally , some of the organisms employed for metal removal may be capable of degrading particular contaminants e . g . phenolic compounds may be degraded by desulfobacteria species . the organics treatment step when applied to soil and the like may be carried out either in - situ or ex - situ as appropriate and as determined by the requirements of the metal leaching step . in ex - situ processes a nutrient solution is contacted with the soil following excavation , over an impermeable base , the leachate solution collected and recirculated following aeration if necessary . in - situ processes for the treatment of soil or land contaminated with organics may involve either injecting , or spraying nutrients onto the contaminated area , thus avoiding excavation . where aeration of the nutrient solution is necessary for contaminant degradation , air may be injected into the contaminated area , or an oxidizing agent , e . g . hydrogen peroxide may be added to the nutrient solution . leachate may be collected either in trenches , or using a system of recovery wells and recirculated . the present invention therefore beneficially allows metal and organic contaminants to be removed from a contaminated medium using a single multistep biotreatment system . as the sulfur source may be at least partially recycled , thus allowing re - use of the process liquor , the process can conveniently be operated as a cyclical system . the present invention offers the following further advantages over prior art processes : ( 1 ) it provides a permanent solution to the contamination problem . ( 2 ) it allows the simultaneous treatment of metal and organic contaminants . ( 3 ) in - situ and ex - situ treatment systems can be available and selected as appropriate . ( 4 ) the size of the secondary waste streams and therefore the cost of dealing with them is minimized . ( 5 ) it minimizes the use of harsh chemicals which could harm the environment . ( 6 ) an opportunity to re - use certain metal contaminants is offered . while various embodiments of the present invention have been described in detail , it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art . it is to be expressly understood that such modifications and adaptations are within the scope of the present invention , as set forth in the following claims . embodiments of the present invention will now be described by way of example with reference to the accompanying drawings , in which : fig1 is a diagrammatic cross - sectional view of a region of land being treated in - situ by a remediation process embodying the present invention together with equipment used in the process . as shown in fig1 a region of land to be treated comprises a layer 1 of soil overlying an underground aquifer 3 below a level 2 . the layer 1 incorporates a metal contaminated region 4 which has been produced by migration of contaminants from a waste sump 5 provided in the surface of the layer 1 . the region 4 extends into the aquifer 3 . a monitoring well 6 projects downward through the region 4 to enable measurements on the extent of contamination in the region 4 to be determined . the depth and dimensions of the contaminated region 4 have previously been determined using appropriate known analytical techniques , ground level is indicated by point 18 . nutrients from a nutrient source 22 and , at a suitable stage in the process acid , which may be carried in a suitable carrier liquid , e . g . aerated water , are applied to the base of the empty sump 5 . this application is carried out by a sprayer 7 . this liquid is also applied via appropriately positioned injection wells 8 and through an infiltration gallery 9 , so as to permeate through the material in the contaminated region 4 . the nutrients are initially selected to promote growth of appropriate micro - organisms to provide organics degradation under substantially neutral ph conditions using one or more of the methods described above . the nutrient addition is later modified to promote soil acidification . during this secondary treatment phase , elemental sulfur may also be added to and mixed into areas of shallow contamination such as the base of the sump 5 further to promote in - situ bioleaching of metal species . in order to enable aerobic conditions to be developed and maintained within the contaminated region 4 , air is blown by an air blower 21 attached to a series of vent wells 10 , ( one of which is shown ) either to draw air through the contaminated region 4 in the layer 1 or to inject air into the ground water in the aquifer 3 or both . additionally , the rate of nutrient addition may be varied to avoid or create anoxic conditions within the contaminated region 4 as appropriate . the plume or region in the layer 1 and aquifer 3 supplied with nutrients and ingredients in an aqueous medium is indicated by reference 20 . this plume 20 encompasses the contaminated region 4 in the layer 1 and aquifer 3 . this treatment degrades organics and subsequently also produces acid metal leaching in the region 4 in the manner described above . this may continue over weeks or months until the soil in the contaminated region 4 is substantially free of contaminating organics and metals as determined from time - to - time by suitable analysis . the products of both organics degradation and metal leaching are collected within a portion of a ground water flow in a direction x , either naturally occurring or artificially created , and are collected by and returned to the surface above the layer 1 via a series of recovery wells 11 ( one shown ) using appropriate pumps ( not shown ). the level 2 of the aquifer 3 may be adjusted by addition of water through an infiltration gallery 24 to assist water flow in the direction x . the collected liquor is then delivered to a selected one of three locations , viz : ( a ) a buffer tank 12 for aeration and addition of appropriate nutrients before re - application to the contaminated area . this is the principal route during initial operation of the process ; ( c ) a gas liquid contactor 14 to scrub hydrogen sulfide from the gaseous effluent from bioprecipitation . liquor enters the reactor 13 at its base and flows upward through the reactor 13 . as it does so , sulfate reducing organisms present in the reactor 13 convert the influent sulfates to sulfides in the manner described above . the gaseous effluent produced during bioprecipitation in the reactor 13 is passed through the gas - liquid contactor 14 connected to the reactor 13 . the contactor 14 permits hydrogen sulfide recovery . the gas stream leaving the contactor 14 is passed through a secondary scrubber unit 19 and discharged to atmosphere . bioprecipitated sludge containing insoluble sulfides is collected in the base of the reactor 13 and transferred via a pipeline 15 to a separate treatment process , e . g . biologically enhanced metal fixation , or is dewatered and collected and delivered to another site for metal recovery . the liquor obtained by dewatering the sludge may either be returned for re - use in the metal bioleaching step of the process embodying the invention , or further treated and discharged . the effluent liquor containing dissolved sulfides arising from bioprecipitation is extracted and combined with the aqueous sulfide stream arising from the gas / liquid contactor 14 . the combined aqueous sulfide stream is then pumped through a gas / liquid contactor 16 and into a sulfide oxidation reactor 17 . the contactor 16 ensures that any gaseous hydrogen sulfide released by acid in the reactor 17 is re - dissolved by the alkaline influent liquor . within the oxidation reactor 17 , the sulfide containing liquor is intimately mixed with suitable micro - organisms and oxidized to sulfate in the manner described above . the acid liquor produced is then transferred to the buffer tank or bioreactor 12 where further elemental sulfur may be added from a source 23 if required , and oxidized to sulfuric acid , by micro - organisms carried over from reactor 17 before re - addition to the contaminated material in the soil 1 in the manner described above ( via the wells 8 and gallery 9 and sprayer 7 ). the added sulfurous material and nutrients forms a plume 20 . the metal removal treatment process is therefore cyclical and metal contaminants in the portion 3 of the soil layer 1 are , during various cycles of the metal removal process , gradually leached by the influent solution containing biochemically formed sulfuric acid and recovered as an insoluble sulfide formed in the bioprecipitation reactor 13 . a proportion of the sulfur is recovered by oxidation of sulfides in the oxidation reactor 17 and is re - used in the soil acid leaching of metal contaminants .