Interactive comment on “ Environmental soil quality index and indicators for a coal mining soil ”

Assessment of soil quality is one of the key parameters for evaluation of environmental contamination in the mining ecosystem. To investigate the e_ect of coal mining on soil quality, opencast and underground mining sites were selected in the Raniganj Coafield 5 area, India. The physical, chemical, biological parameters, heavy metals, and PAHs contents of the soils were evaluated. Soil dehydrogenase (+79 %) and fluorescein (+32 %) activities were significantly higher in underground mine (UGM) soil, whereas peroxidase activity (+57 %) was higher in opencast mine (OCM) soil. Content of As, Be, Co, Cr, Cu, Mn, Ni, and Pb was significantly higher in OCM soil, whereas, Cd was 10 higher in UGM. In general, the PAHs contents were higher in UGM soils probably due to the natural coal burning in these sites. The observed values for the above properties were converted into a unit less score (0–1.00) and the scores were integrated into environmental soil quality index (ESQI). In the unscreened index (ESQI-1) all the soil parameters were included and the results showed that the quality of the soil was 15 better for UGM (0.539) than the OCM (0.511) soils. Principal component analysis was employed to derive ESQI-2 and accordingly, total PAHs, loss on ignition, bulk density, Be, Co, Cr, Ni, Pb, and microbial quotient (respiration: microbial biomass ratio) were found to be the most critical properties. The ESQI-2 was also higher for soils near UGM (+10.1 %). The proposed ESQI may be employed to monitor soil quality changes due 20 to anthropogenic interventions.


Introduction
Coal, a combustible rock rich in carbon, is a crucial component of the energy mix that fuels the globe. In many countries, more than 70 % of the electricity generation comes from coal. For more than 150 years, coal has been an important source of energy for both developing and industrial societies. Coal mining is one of the core industries and Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | environmental sustainability (Masto et al., 2007a). Integrated soil indices based on a combination of soil properties provide a better indication of soil health than individual properties. Moreover an integrated index is essential for quantitative comparison of different soils. Several indices have been proposed to assess soil quality, which were mostly microbial in nature and for agricultural soils. Indexing involves three main 5 aspects: (1) choosing appropriate indicators for a minimum data set, (2) transforming the indicators to scores; and (3) combining the scores into an index (Sinha et al., 2009). Soil quality indices are useful to differentiate between degraded status of soils (Morugán-Coronado et al., 2013). Studies on soil quality indices involving soil contaminants are limited. Thus, the present study was aimed to assess the physical, 10 chemical, biological parameters along with heavy metal and PAH contents of soils near an opencast and underground coal mine. The other objective was to integrate all these parameters into a comprehensive environmental soil quality index (ESQI). 15 Raniganj Coalfield is primarily located in the Asansol and Durgapur subdivisions of Bardhaman district, West Bengal, India. Raniganj Coalfield covers an area of 443.50 km 2 and has total coal reserves of 8552.85 million t. Eastern Coalfields (ECL) reported its reserves as 29.72 billion t that make it the second largest coalfield in the country (in terms of reserves). Surface soil samples (0-0.15 m depth) were 20 randomly collected from the settlements near an opencast mine (Sonepur Bazari) and underground mine (North Serasole) of Raniganj Coalfields. All together 32 samples were collected from the opencast mining (OCM) site and 17 samples from underground mining (UGM) site. A portion of fresh soil samples were refrigerated for analysis of soil biological parameters. The rest of the samples were air dried ground and passed 25 through 2 mm sieve for further analysis.

Soil analyzes
The methods described by Tandon (1993) and Baruah and Barthakur (1999) were used to determine the following soil properties: bulk density (BD) (soil core method), maximum water holding capacity (by equilibrating the soil with water), porosity (derived from bulk density), pH and EC in water (1 : 2.5, soil / water ratio), soil organic carbon 5 (by potassium dichromate oxidation), and loss on ignition. Active microbial biomass carbon (AMBC) was measured by the glucose nutrient induced respiration method (Islam and Weil, 2000). Soil dehydrogenase activity was determined using the method of Klein et al. (1971). Phenol oxidase and peroxidase were measured with L-DOPA (L-3, 4 di hydroxy phenyl alanine) as substrate in acetate buffer (Robertson et al., 10 1999). Basal soil respiration (BSR) was measured as the CO 2 evolved from moist soil, adjusted to 60 % WHC, over an incubation period of 10 days at 25 ± 1 • C, in the dark (Islam and Weil, 2000). Soil metabolic quotient (AMBC/SOC) was calculated. Specific maintenance respiration rates (qCO 2 ) were calculated as BSR per unit of active (BSR/AMBC) microbial biomass carbon (Anderson and Domsch, 1990;Islam 15 and Weil, 2000). Phosphatase enzyme (p-nitrophenyl phosphate method, colorimetry); fluorescein diacetate hydrolase activity (FDA) of the soil was determined by the method described by Dick et al. (1996). For analysis of soil heavy metal content, the soil samples were digested in a microwave (ETHOS, Milestone, Italy) as per USEPA 3051A method (USEPA, 2007) and filtered. The metals in the filtrate were determined by ICP-20 OES (iCAP 6300Duo, Thermo Fisher Scientific, UK). For soil PAH analysis, samples were extracted using 1 : 1 hexane: acetone mixture in microwave as per the USEPA method 3546 (USEPA, 1995). The concentrated extract was analyzed by GC-MS system (Varian 450 GC and 240 MS) for 16 PAHs. 25 Two types of indexing system was followed to derive the environmental soil quality index (ESQI).

Environmental soil quality indices
Where, S denotes score of observed soil parameter, n is the number of parameters included in the index.

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Principal component analysis (PCA) was used to select the appropriate properties and their weighing factors. PCs with eigen value ≥ 1 and explained at least 5 % of the variation of the data are examined (Sharma et al., 2005). Under a particular principal component (PC), only the variables with high factor loadings were retained for indexing. High factor loadings were defined as having absolute values within 10 % 10 of the highest factor loading (Andrews et al., 2002a). When more than one variable was retained under a single PC, multivariate correlations were employed to determine if the variables could be considered redundant and, therefore, eliminated from the ESQI (Andrews et al., 2002b). If the highly loaded factors were not correlated then each was considered important, and thus, retained in the ESQI. Among well-correlated 15 variables, the variable with the highest factor loading (absolute value) was chosen for the ESQI. Each PC explained a certain amount of variation (%) in the total data set; this percentage provided the weight for variables chosen under a given PC. The final PCA based ESQI equation is as follows:  (Sinha et al., 2009), with an asymptote tending to 1 and another tending to 0.
Where x is the soil property value, a is the maximum score (= 1.00) of the soil property, x 0 is the mean value of each soil property, b is the value of the slope of the equation.

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The slope was −2.5 for the "more is better curve" and +2.5 for the "less is better curve" to obtain a sigmoidal curve tending to 1 for all the proposed properties.

Statistical analysis
The data were expressed as mean values and compared statistically by t test; P significance is presented. The ESQI was done using PCA (Andrews et al., 2002b). 10 For computation, SYSTAT-12 package was used.

Basic soil properties and biological parameters
Soil bulk density, porosity, water holding capacity, pH, and electrical conductivity were not significantly differed between the OCM (open cast mine) and UGM (underground 15 mine) soils ( and fluorescein (+32 %) activities were significantly higher in UGM soil, whereas peroxidase activity (+57 %) was higher in OCM soil. Different soil may inactivate enzyme reactions by complexing the substrate, by reacting with protein-active groups of enzyme-substrate, or by reacting with the enzyme substrate complex or indirectly by altering the microbial community, which synthesizes enzymes. Enzyme activity may 5 either increase or decrease due to environmental contaminants. Heavy metals affect microbial metabolism by altering the normal enzyme activities, particularly inhibition of a specific enzyme and the effects can be dramatic and systemic (Christensen et al., 1982). The presence of different heavy metals and PAHs in coal contaminated soils might have altered the soil enzyme activities (Masto et al., 2007b).
Most of the elements are enriched in OCM soil probably due to the relatively higher land disturbances and coal dispersion in OCM site. Among the elements, the content of Co and Cr was > 50 % higher in OCM soils, these elements might have originated from 20 the coal. The rate of release of Cr into the global atmosphere from coal combustion is estimated to be in the order of a few thousands of tons per year. The mean Cr content of coals is only 20 mg kg −1 worldwide (Huggins et al., 2000).
Cd was slightly higher in the UGM soils. In Yatagan, Turkey, Yapici et al. (2006) reported that exploration of coal minerals contributed for Cd concentration in the local 25 biota. The Zn content was not affected significantly between the OCM and UGM sites probably the soil Zn is originated from vehicular activities. Tyre treads and tyre dust  et al., 2011), thereby it is likely that the contamination of both OCM and UGM soils with Zn is from vehicular activities.

Soil PAHs
Among the soil PAHs, acenapththylene and phenanthrene were not significantly affected between OCM and UGM soils (Table 3). Naphthalene (+26 %), fluorene (+66 %), 5 anthracene (+43 %), fluoranthene (+44 %), pyrene (+52 %), benz(a)anthracene (+41 %), chrysene (+50 %), benzo(b)fluoranthene (+66 %), benzo(k)fluoranthene (+69 %), benzo(a)pyrene (+62 %), and total PAHs (+24.3 %) were significantly higher in UGM soils. Acenaphthene (+43 %), benzo(g,h,i)perylene (+89 %), and dibenzo(a,h)anthracene (+94 %) were significantly higher in OCM soils. In general the PAHs contents were higher in UGM soils probably due to the natural coal burning in these sites. The said coal mine has experienced mine fires and mining operations were closed for quite a few years. Tsibart et al. (2014)  180 mg kg −1 ), but the high-molecular-weight PAHs (benz(ghi)perylene, benz(a)pyrene, benz(k)fluoranthene) were revealed only in charry peat horizons. During coal burning the organic compounds in the coal are partially cracked to smaller and unstable fragments. These fragments, mainly highly reactive free radicals with a very short average lifetime, lead to more stable PAH formation through recombination reactions 20 (Mastral and Callén, 2000). Further, fluoranthene and pyrene are enriched in UGM soils, and are commonly considered as typical pyrogenic products derived from high temperature condensation of lower molecular weight aromatic compounds (Li et al., 2010). PAHs are emitted in the gas and solid phases. Both these PAHs can travel in the atmosphere and settle down on soil, water bodies and other environmental media. 25 Natural mine fire as well as domestic use of coal for cooking in the UGM site might have contributed to elevated PAHs content in the UGM soils. Natural coal fires were not reported in the OCM sites of the present study. Domestic coal burning is also not present in OCM site as the OCM is away from residential area.

Environmental soil quality index
Individual soil parameter values were normalized on a scale from 0 to 1 based on two types of curve: "more is better" (POR, WHC, and pH. SOC, DHA, FDA, AMBC, BSR, 5 PHOH, POH, ACP, AKP, AMBC/SOC); "less is better" (BD, EC, LOI, heavy metals, PAHs, BSR/AMBC). More is better was designated for pH as the mean pH in both these soil was < 7.0. Less is better was designated for loss on ignition, as it indicates the quantum of coal contamination. The calculated scores were integrated in to ESQI by two indexing methods as below.

Unscreened transformation (ESQI-1)
The index is the summation of the scores obtained by individual indicators, divided by the total number of indicators, here all the soil parameters has equal weightage.

Principal component analysis based index (ESQI-2)
All the soil variables were included for principal component analysis. The first five PCs had eigenvalues > 1.00 (Table 4). The highly loaded variable under PC-1 was total PAHs and was included in the ESQI. Likewise in PC-2, LOI, dibenzo(a,h)anthracene, indeno(1,2,3,c,d)pyrene, and benzo(g,h,i)perylene were highly loaded. As these 5 parameters are highly correlated (r > 0.700) among themselves and the total PAHs were already included in the ESQI, only LOI from PC-2 was included in the index. Similarly Be, Co, Cr, Ni, and Pb from PC-3 was included from PC-3, all these elements were correlated among themselves, therefore the weight corresponding to PC-3 was equally divided among these elements. BD, POR, and WHC were highly loaded from This is probably one of the first studies where total PAHs has been used as a soil quality indicator. In line with PAHs, the LOI was also included in the ESQI. The LOI could be an indirect measure of the coal contamination in the soils, the LOI observed in the soils is much higher than the normal soils. As coal is an organic matter, it contributes to soil 20 LOI and PAHs. The soil PAH is very important in contamination and human exposure point of view. PAH profile in UK soils, indicates that benzo[a]pyrene is a good surrogate marker, being ubiquitous in sites contaminated with PAHs and providing a consistent indicator of the amount of PAHs in contaminated soil (HPA, 2010). Fluoranthene is suggested as a complementary indicator to benzo(a)pyrene (Bostrom et al., 2002).
Heavy metals are ubiquitous pollutants coal has also been reported as one of the source of heavy metal pollutants especially Cr, Ni, and CO ( some of these elements are enriched in the coal. The bulk density is an important parameter which affects the soil productivity of the coal-mined land because it indicates the suitability of the soil for root proliferation, water-holding capacity, and long-term nutrient availability. It is found to affect the entire biological reclamation process by influencing moisture retention capacity, and porosity. The ratio BSR / AMBC is most 5 appropriately used as an index of adversity of environmental conditions for the soil micro flora and has valuable applications as a relative measure of how efficiently the soil microbial biomass is utilizing carbon sources, and the degree of substrate limitation for the soil microorganisms (Wardle and Ghani, 1995). Soil microorganisms divert more energy from growth to maintenance as stress increases and thus the ratio of respired 10 C to biomass C can be a much more sensitive indicator of stress. The BSR / MBC ratio indicates the carbon turnover rates in the soils, the importance of soil organic carbon in improving the overall soil quality has been reported widely (Debasish et al., 2014;Fialho and Zinn, 2014; Lozano-García and Parras-Alcántara, 2014). Paz-Ferreiro and Fu (2013) reviewed the limitations of using soil biochemical, microbiological, and 15 biological properties for soil quality evaluation. The ESQI-2 obtained using the PCA is presented in Fig. 1b, where the contribution of each soil indicator parameter on calculated ESQI is also shown, which gives an insight into the cause for the measured ESQI. The ESQI is higher in UGM (+10.1 %) than OCM soil. Total PAHs and LOI are the limiting parameters in UGM (−39.6 %) and 20 OCM (−143 %) soils, respectively.
In general, the UGM has comparatively less environmental impacts than opencast mining. As underground mining operations take place below ground, they generally do not create as much air pollution, contribute less to groundwater, surface water and soil pollution, and are not as visually intrusive. Similar soil quality studies on mining area 25 showed that reclamation of mine soil through plantation could improve the SQI score (Asensio et al., 2013).