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Details from the Discussion of Tasmanian paper:
4. Discussion and conclusions
Unbleached toilet paper does break down faster than bleached toilet paper and tissues. However, tampons stand out as being most resistant to decay, with the other products not strongly differentiated in their rates.
The sites that recorded the greatest decay rates were those that were warm, relatively dry and not acidic (Table 5). Breakdown of most products was well advanced within 6 months of burial at these sites. Microbial activity, measured by cellulase assay, was also greater at these sites (Bridle et al. unpublished data). Line (1998) found that the cellulose flocking used in disposable nappies decayed after 5 months in warm environments with neutral, fertile soils.
The independent variables that were incorporated in the models of decay (Table 4) are largely consistent with those that are associated with peat formation, which occurs in cold and/or waterlogged and/or acid places (Moore and Bellamy, 1974). Indeed the two sites that exhibited little decay after 24 months (montane moorland and western alpine) had organic soil profiles, while the others had mineral soils with a surface organic horizon of varying depths. The importance of cations (both measured directly, and indicated by pH), relatively dry soils and high temperatures in promoting disintegration has been noted for cotton strip assays in a wide variety of environments (Harrison et al., 1988). Higher decay rates during the late summer–autumn period than during the spring–early summer period has also been reported elsewhere (French, 1988). However, our data show that three sites (coastal eucalypt, subalpine rainforest, montane moorland) recorded greater decay in the cooler months than in the warmer months for the fertilised bags. There was no significant difference for decay within the same treatments but between times for any of the other sites. The difference in decay between the seasons is likely to be related to precipitation patterns during the sampling times. The autumn–winter bags were buried in 2000, while the spring–summer bags were buried in 2001–2002. There was a severe drought during 2000, and heavy summer rainfall during the spring–summer of 2001–2002. Rainfall is an important indicator of decay for fertilised bags (Table 4). The greater decay in the drier period is consistent with our model (Table 5).
Depth of burial is an important factor in decay at sites where soils freeze (Lawson, 1988). Under such conditions, there is a difference in decay with depth down the soil profile during the early summer, though this difference is not evident later in the season. Our data show depth of burial to be largely unimportant across all sites in Tasmania, but in wetter areas where water tables are within 15 cm of the surface, paper products are likely to decay more readily at 5 cm depth than at 15 cm depth. While decay may be slightly enhanced at shallower burial depths, access of faeces to native animals and transport of faecal bacteria may occur more readily at 5 cm depth than at 15 cm depth. The burial of waste under rocks at the soil surface does not increase decay, and is inadvisable from a public health point of view (Bridle et al., 2003).
It has been shown that the addition of nutrients via sewage sludge enhanced decomposition of cotton-strips compared to an untreated control soil (Obbard and Jones, 1993). The addition of both N and P to cotton strip assays in an Everglades marsh led to greater decomposition than was recorded for the addition of only one of the elements, especially where the strips were buried in the peat layer rather than in the water column (Maltby, 1988). There was no relationship between C/N ratios and breakdown success. This finding has also been documented for the decay of coarse woody debris in forest environments (Mackensen et al., 2003).
Research into the decay of coarse woody debris in forest environments showed temperature to be an important determinant of decay (Mackensen et al., 2003), but initial density of the wood was also important. These results can be related back to tampon decay, as tampons are much denser than toilet paper or tissues.
Mackensen et al. (2003) found a relationship between annual rainfall and decay rates, where decay was less at sites that received more than 1300 mm of rainfall. Ineson et al. (1988) suggested that that potential evaporation could be a good predictor of cellulose decay. We have developed a simple index of decay that is also based on temperature, rainfall and pH. The index is derived from two classes of mean annual precipitation (> 650 mm=1, < 650 mm=2), mean annual temperature (<13 °C=1, >13 °C=2) and pH (<4.5=1, >4.5=2). This index reconstructs the order of mean decay using all 6 month autumn–winter, 12 month and 24 month data (Table 5). A score of three on this index indicates that 2 years is insufficient for decay of all paper products buried in the soil, even when fertilised with faeces or urine. A score of six indicates a rapid dissolution of products, whether fertilised or not. This index may be exportable to other parts of the world.
4.1. Implications for management
A key question in deciding the implications for management of our decay data is the social and environmental acceptability of different periods of persistence of human waste disposal products in the soil. Social acceptability relates to the probability of excavating the evidence of a past faecal burial event, when undertaking preparations for a new event. This probability can be high in some well-used places (von Platen, 2002; authors unpublished data). Environmental acceptability relates to variation from the natural condition of the soil, which would obviously be considerable where deposits remain intact over several years. In the western alpine and high altitude moorland environments decay is extremely slow. In our judgement it is both socially and environmentally undesirable to continue to advise people to bury their wastes in these environments. This would not be a major imposition on walkers, as locations in these environments are usually in close proximity to forest or scrub vegetation, which provide more privacy than buttongrass moorland and alpine vegetation.
If anything is to be carried out, tampons are an appropriate target. Current MIB prescriptions request that tampons are carried out and not buried in the ground. As this is a simple and broad-ranging message with little public health risk to the walker, we suggest that the message is retained, despite the relatively successful decay of tampons, albeit after 2 years at some of the sites.
Walkers may place their wastes under rocks in alpine areas because they are reluctant to damage alpine vegetation by digging. We hope that the results of this research and those from our vegetation study (Bridle and Kirkpatrick, 2003) will convince them that it is less environmentally harmful to bury their waste than to leave it exposed.
Soil depth proved sufficient in parts of all our sites to enable burial of waste at 15 cm, as is suggested by the code. However, obstructions such as roots, rocks or very hard clay soil made it difficult to dig a hole 15 cm deep at some sites. Digging to that depth was impossible to severely challenging at most sites using plastic trowels of the kind sold in many outdoor stores. There is a need for prescriptions in the MIB guidelines on the strength and quality of trowels. Burial at 5 cm does present some relatively low chance of excavation by animals, compared to 15 cm, so the 15 cm recommendation in the code should stand.
Recent research which aimed to determine the impact of the addition of real faeces and urine on toilet paper decay, showed similar responses over a one year period to the results we have detailed above. Results from two extreme sites (coastal eucalypt and montane moorland) were consistent with the data presented in this paper (von Platen, 2002). While the presence of faeces may have allowed additional bacteria to survive in the environment, toilet paper decay was not significantly enhanced in a 6-month-period.
The above results suggest that the minimum impact bushwalking code should be amended to: (1) to recommend no disposal of faeces, toilet paper or tissues in treeless vegetation above 800 m in western Tasmania; (2) to emphasise that placement of waste under rocks causes more environmental harm than disposal by burial, even in alpine environments; (3) to emphasise that strong metal trowels are necessary to excavate holes for defecation in most wild places. The significantly longer decomposition times for tampons compared to toilet paper supports the current policy of carrying out tampons.
Guidelines should also advise walkers to choose their toilet site carefully. Choose a well-drained soil in woody vegetation rather than a poorly drained soil or peat in alpine or moorland vegetation.
The index we derived for predicting the speed of decay of human waste disposal products requires testing outside Tasmania, to determine its potential universality.
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