A less common leaching test is a Field Leaching Test. This involves the construction and operation of small scale disposal sites, based on the expected design of an operational site. There are only a small number of published field studies available in the literature, due principally to their expense to construct and lengthy period of operation. The thickness, width, leachate drainage and collection system, method of inflow application (eg. artificial irrigation or rainfall), period of sampling, internal moisture and chemistry monitoring and so on are variables which depend on the objectives to be achieved.
Fruchter et al. (1990) presented the results of 3 years of monitoring and geochemical analysis at an alkaline fly ash field lysimeter at a power station in Pennsylvania, USA. The above-ground lysimeter was 3 m in height, and built as a truncated pyramid with a 30.5x30.5m base and 18.3x18.3 m top. A 50 cm layer of coarse bottom was placed at the base to facilitate drainage. The only inflow was rainfall, leaving the lysimeter with unsaturated conditions for most of the year. The leachate was generally only produced in the winter months from October to May, after heavier rainfall events. The cumulative leachate over the 3 years amounted to 0.3 pore volumes. The monitoring data and geochemical analysis showed that the major solutes leached from the ash were Ca and SO4, controlled by dissolution of gypsum. Low concentrations of Fe and Al were detected, and were considered to be controlled by their respective amorphous hydroxides. The presence of silica (SiO2) in the leachate was observed, although its mineral source could not be identified with certainty. There were several trace elements detected in leachate at varying concentrations, namely As, Ba, B, Cd, Cr, Cu, Mo, Sr and Se. Most trace elements appeared to be controlled by solubility and complex co- precipitation processes, although for As, B, Cd, Mo and Se geochemical controls could not be identified.
Hasselriis (1994) presented the results of a field study of an 18 m high (about 60 ft) ash stockpile subjected to natural rainfall for between 4 to 7 years. The ash was derived from a municipal solid waste incineration facility in the USA. The annual rainfall was approximately 100 inches or 2,540 mm. A series of TCLP tests were conducted on core samples of ash obtained from the stockpile. Only the data for lead and cadmium is presented in this paper, and hence comments are limited to these elements. The leachability of Cd was generally very low, at less than 20% of the regulatory criteria. For Pb, the leachability data appeared to indicate substantial migration due to the infiltration of rainwater through the ash profile, leading to redistribution and concentration in the profile. Hasselriis (1994) concluded that ash leachability was low.
Igarashi & Shimogaki (1996) reported on a similar 2 year lysimeter study undertaken in Japan to investigate the reclamation of alkaline coal ash disposal sites. Their primary focus was on modelling of leachate production and migration. The lysimeter used a steel container 5 m in length, 1.5 m in width and up to 2 m in height. An 80 cm layer of sand was placed in the base of the lysimeter for drainage purposes and a 40 cm of loam soil was used to cap the lysimeter. A vertically installed sheet pile was used to control leachate flow during operation. Seawater was then irrigated on the surface until a flow rate of about 83 litres per day was achieved. No fresh water was added to simulate rainfall. The evapotranspiration rate was much lower than the flow rate and this was therefore ignored in the water budget. The monitoring data showed leaching of Ca and Cr from the ash layer into the sand layer, which discharged to the sea. The modelling of the lysimeter was based on unsaturated flow theory using the FEMWATER numerical model. The leaching of ash was represented by a two-fraction approach, with the leaching governed by a first-order kinetic reaction with respect to the solid phase concentration for each fraction, shown below. The two-fraction leaching model was able to represent the overall flow and solute leaching behaviour in the lysimeter, correlating with the transport of chromium through the drainage layer to the sea.
1 1 1 .S k t S = ∂ ∂ 2 2 2 .S k t S = ∂ ∂ and S1 + S2 = S0 2-1 where S1 - rapidly-leaching fraction of solid phase concentration;
S2 - slowly-leaching fraction of solid phase concentration; S0 - total solid phase concentration;
k1 - first-order rate constant for the rapidly-leaching fraction; k2 - first-order rate constant for the slowly-leaching fraction.
Johnson et al. (1998) reported an extensive field study of an operational landfill in Lostorf, Switzerland, accepting bottom ash from a municipal solid waste incinerator. Their study was conducted over a period of 22 months and examined rainfall, leachate discharge, dye tracer tests and oxygen stable isotopes (18O/16O). Their work showed strong evidence of preferrential pathways for water migration in the landfill, leaving little time for interaction with and leaching of the contained soluble salts in the bottom ash. They did not discuss or assess what these preferrential pathways could be. On the basis of chemical and isotopic analyses, they calculated that about 9 to 40% of summer discharge was derived from ash leaching, showing a high proportion of rapid preferrential flow due to the higher intensity rainfall in summer (that is, 60 to 91% of discharge being rainfall). About 20 to 80% of the incident rainfall passed through the landfill in summer. In winter, about 90 to 100% of the discharge was leachate, with only about 10% of the rainfall passing through the landfill. They described this effect as "piston flow", and was critical in examining the time for geochemical reactions in the ash and whether equilibrium could be assumed in hydrological modelling. Their data suggested that a "quasi-equilibrium" situation existed at Lostorf, allowing realistic geochemical modelling of leachate generation and solute transport within the landfill.
The authors did not discuss the hydraulic mechanisms controlling the flow of moisture within the ash landfill and the respective proportions of leachate over the two years studied. The data they present suggests strong control due to unsaturated conditions prevailing within the ash profile, related to the intensity of inflow. This behaviour is important and will be examined further in later chapters of this thesis.