The degradation of plastic litter in rivers: implications for beaches -
polymer losses of between 1% and 3% were the most that could be expected. Seal (1988) considered that after exposure to UV-light, biodegradation of the remaining polymer was not enhanced, but that small molecules produced by photo-oxidation were the only biodegrad- able element which could decompose to give the overall polymer loss. The lack of naturally occurring biodegra- dation of plastics has stimulated intensive research with a view to producing truly biodegradable polymers. Al- though polyethylene plastic has been manufactured which exhibits increased photochemical oxidation and therefore fragments more quickly, the packaging cannot be considered truly degradable until fragments undergo further decomposition to components which may recy- cle in nature.
Onions & Rees (1992), in an investigation of photodegradable Hi-Cone carriers (4/6 pack holders) in the South Wales beach environment, demonstrated that use of photodegradable plastic resulted in earlier embrittlement and fragmentation. Tensile testing was carried out on samples of conventional and photo- degradable Hi-Cone carriers after various exposure tri- als. Ultimate embrittlement (reduction to 5% elonga- tion) of the photodegradable samples was reached after only 74 days on a UK beach (Merthyr Mawr, Mid- Glamorgan), whilst no reduction in elongation was re- corded for the conventional carriers. Tensile testing, as a measure of degradability, produced conclusive results in the Hi-Cone study. The benefits of this uncompli- cated test procedure and its application to environmen- tal trials led to its adoption for use in this research as a measure of polyethylene degradation in riverine envi- ronments pre deposition on a beach.
The river and beaches
The Severn estuary and Bristol Channel system is an aggressive morphogenic environment with macro-tides (14.8 m at Avonmouth) and is subject to frequent storms, especially in the SW direction. Many rivers discharge into the Channel with estimated sediment inputs varying from 0.7 105 tonnes per year (Brookes 1974) to 1.8 106 tonnes per year (Shaw 1977). Selected sites were established in rivers and assessed for baseline surveys by the authors. To ensure optimum litter deposition conditions (Dixon & Dixon 1981) low energy, prefer- ably sandy, beaches with wide reach zones and multiple strandlines were chosen (West Aberthaw, Llantwit Ma- jor, Southerndown, Merthyr Mawr). Similarly, 50 river- bank sites were analyzed for litter over one year on the River Taff, South Wales. Some 22 % of the river Taff’s litter is made up of feminine hygiene products, litter items averaging 584 items per kilometre of bank
(Simmons & Williams 1994).
The Irish Sea Study Group (Anon. 1990b, p. 52) concluded from numerous beach surveys that “approxi- mately 50% of plastic containers were believed to origi- nate from ships discharges, and the remainder from land-based sources, primarily holiday makers”. This does not appear to be the case in the South Wales region. High numbers of beverage, dairy product and DIY- related containers indicates greater contribution from land-based sources, either by beach users or riverine inputs. This result is not surprising considering the Bristol Channel’s estuarine nature, the comparative lack of shipping and the unlikely occurrence of major oce- anic inputs. Analysis of container origins indicated ship discards were less prominent in the Bristol Channel survey areas, the geographical origin being 92 % Brit- ish. By number, 82 % of containers reported in this paper on these estuarine beaches were plastic, 17 % metal and 1 % glass. Many of these plastics were felt likely to have originated from riverine fly-tipping sites. Widespread illegal dumping of all types of waste, but in particular that of household origin, has led to the estab- lishment of numerous fly-tipping sites along the banks of the River Taff. During high-flow conditions, mobile objects such as containers were often observed being carried from such sites in the river flow, but no increases were recorded at riverine survey sites when water levels receded, suggesting transportation to the sea. Plastic containers considered to be DIY-related, for example interior paints, herbicides and carpet cleaners were thought unlikely to have originated from either shipping vessels or beach-users and thus were likely to be of riverine origin. For non-containers, 58% of the beach litter was plastic in origin.
A summary of results obtained from a 5-m wide transect at Merthyr Mawr beach, Wales, on December 16, 1994 showed the following:
of 118 metal items, 94 were aluminium cans;
25 glass items were bottles (mainly wine);
sweet wrappers/crisp packets constituted 240 items
out of 272 paper items found;
out of 97 sanitary items counted, 65 were sanitary
towel backing strips;
out of 455 plastic items, 114 were plastic sheeting and
96 were bottles.
These are extremely high figures that have been borne out by many surveys in this area. The NUC (Anon. 1995) study of beach strand lines in South Wales found that sanitary items averaged 11 per km; metal cans 115 per km and plastics bottles 128 per km. These items are essentially riverine in origin and surveys car- ried out by the authors on the same estuarine beaches gave quite different result (Williams & Simmons in press).