Plant community and soil PAH pollution survey
Quadrat sampling of desert plant communities (4 m2quadrat unit size; n = 200 samples) was carried out in 2011 within the seven sampling areas. The selected areas represent a range of geographical locations and intensity of soil PAH pollution throughout Kuwait.
The fieldwork was carried out from January to March 2011. This sampling period was at the end of the rainy season in which the soil seedbank generally germinated after rain events and provided an opportunity to carry out ecological study of desert plant communities (Springuel et al. 1996, 1997; Bainbridge 2007). The number of samples varied at different areas, but each sampling event per area contained at least 10 quadrats. The Universal Transverse Mercator (UTM) geo-coordinates were determined using GPS device (Garmin Etrex: Basic), and minimum of 5 randomly positioned quadrats were sampled around the established sampling site. The percentage frequency of individual species, and mean vegetation height (cm) at each quadrat were collected. The nomenclature of plant species followed the procedure used by Boulos & Al-Dousari (1994). We further confirmed the naming of the species using The Plant List (2013):www.theplantlist.org.
Species abundance was assessed using a quadrat-scale frequency method. For each quadrat, the percent frequency of individual species (%F), and the mean vegetation height (cm) were recorded to determine plant distribution within the selected areas. Species abundance was assessed using the percent frequency at two scales. The first scale was at the sample area scale calculated as:
\begin{equation} \%FA=100\times\frac{\text{number\ of\ quadrats\ with\ species}}{\text{total\ number\ of\ quadrats\ sampled\ per\ area}}\nonumber \\ \end{equation}
The second scale was at the quadrat scale. for every species present in the sampled quadrat, the %FQ was calculated using the following equation:
\begin{equation} \%F=\frac{number\ of\ hits\ within\ 400\ sub-units}{4}\nonumber \\ \end{equation}
The calculation is based on the fact that each quadrat covers an area of 4 m2, and the plant species count was conducted by dividing the quadrat into 400 sub-units with 10 x 10 cm in each unit.
Several vegetation parameters were also measured including the mean vegetation height averaged across the quadrat (cm), species diversity as number of species present per quadrat sample (SQm-2), and number of species per sampling area (SA). Other variables determined for each sample unit were latitude and longitude (°N; °E), semi-quantitative Oil Damage Score (ODS: on a 1 – 3 scale, with 1 = no visible damage, to 3 = major visible evidence of oil damage to soil), and concentrations of polycyclic aromatic hydrocarbons (PAHs: μg kg-1 dry weight of soil or plant tissue) in soil.
Plant leaves and soil samples were collected to assess the degree of soil hydrocarbon pollution, and to determine the uptake of petroleum hydrocarbons by the chosen dominant plant species. A total of 63 plant tissue (leaf) samples, from three oil-polluted areas (Um Al-Aish, Sabriya, and Um-Ghadair Oilfields) and one uncontaminated area (Sabah Al-Ahmad Protected Area) were collected in sealed glass jars (frozen) and subsequently analysed at the Central Environmental Laboratory (CEL) (ISO 17025), Kuwait University. These samples included 32 H. salicornicum samples, collected from the Um Al-Aish Oilfield, and 18 specimens of C. conglomeratus (16 from the Sabriya Oilfield, one from Sabah Al-Ahmad Protected Area, and one from the Um-Ghadair Oilfield). The other 13 specimens were R. epapposum , from the Sabriya Oilfield.
In carrying out the soil sample analysis, 184 soil samples were analyzed at CEL in March 2011, comprising 35 soils from Um Al-Aish, 30 from Sabriya Oilfield, 26 from Bahra, 7 from Um Al-Rros, 45 from Sabah Al-Ahmad Protected Area, 21 from Burgan Oilfield, and 20 from Um-Ghadair Oilfield. The moisture contents of soil and plant tissue samples were determined, and the samples were then analysed for 16 priority PAHs, identified by the US Environmental Protection Agency (US-EPA) in oil polluted soils. These 16 compounds were acenaphthylene (ANAY); acenaphthene (ANA); anthracene (ANTH); benzo(a)anthracene (B[a]ANTH); benzo(a)pyrene (B[a]P); (benzo(g,h,i)perylene (B[ghi]PERY); benzo(b)fluoranthene (B[b]FLAN); benzo(k)fluoranthene (B[k]FLAN); chrysene (CH); dibenzo(a,h)anthracene (D[ah]ANTH); fluoranthene (FLAN); fluorene (FL); indeno(1,2,3-cd) pyrene (I[123-cd]PY); naphthalene (NA); phenanthrene (PH) and pyrene (PY). In this study, these hydrocarbons fractions were used to assess the total Petroleum Hydrocarbon (TPH: mV.sec) content of oil-polluted soils. The concentrations of each compound are reported as µg kg-1 of dry soil or dried plant tissue. These compounds were quantified using semivolatiles gas chromatography/mass spectrometry (GC/MS) method (Smith & Lynam 2014).