Concentrations Of Polynuclear Aromatic Hydrocarbons Research Articles

Organic and trace metal contaminants in sediments and English sole tissues from Vancouver Harbour, Canada.

As part of a multinational workshop on marine environmental quality, sediments were collected from seven sites in Vancouver Harbour and analyzed for polynuclear aromatic hydrocarbons (PAHs) and organochlorines (OCs), and for trace metals. English sole were collected from five sites, and muscle was analyzed for trace metals and liver for OCs. As expected, sediment PAH and OC concentrations and tissue OC concentrations were higher at sites east of the First Narrows, compared to the outer harbor and reference sites. Sediment PAH concentrations east of the First Narrows were similar to concentrations at moderately contaminated sites in Puget Sound, south of Vancouver Harbour. In contrast, concentrations of OCs in sediments and tissue were low to moderate, even at relatively contaminated sites within Vancouver Harbour. Although several trace metals in sediments were higher than in contaminated sediments from Puget Sound, trace metals measured in fish muscle were lower.

Characteristics of Polynuclear Aromatic Hydrocarbons in Ambient Air through a Long-Term Sampling Program at a Metropolitan City in Taiwan

Polynuclear aromatic hydrocarbon (PAH) concentrations in total suspended particulate (TSP) matter as well as gaseous PAH contents are measured at an urban site adjacent to the capital of Taiwan. Several factors, such as seasonal variation, ring number, G/P (gas/particulate) ratio, and C-atom number, are utilized to characterize the pollution features of 14 PAHs. The results show that the total PAH content in TSP has been reduced gradually. The probable carcinogenic PAH compounds exist primarily in the particulate phase. The concentration distributions of each PAH compound are different, and the quantities and ring distributions of PAHs are significantly affected by seasonal fluctuation. The G/P ratio is highly associated with the C-atom number of PAHs. Factor analysis, along with the characteristic ratios of PAHs, is used to qualitatively identify the probable contributors. The results suggest that traffic exhaust and industrial origins are the predominant contributors.

Monitoring of time variation and effect of some meteorological parameters in polynuclear aromatic hydrocarbons in Ioannina, Greece with the aid of HPLC-fluorescence analysis

This paper describes the monitoring of 16 polynuclear aromatic hydrocarbons (PAH) in the suspended particulate matter in the area of Ioannina. A total of 244 samples were collected from five locations throughout the city. These samples were analyzed in order to acquire a complete picture of the PAH abundance and distribution above the city of Ioannina. The sampling sites covered the center of the city as well as the suburban territory. The monitoring lasted for one year starting in November 1996. Higher values of PAH were found at the sites surrounding the city center, while at the rest of the area the concentration of PAH was substantially lower. The effect of ambient temperature and humidity on PAH abundance and distribution was also evaluated. As a result it was revealed that higher concentrations of PAH are favored by low temperatures and high values of humidity. Regarding the seasonal variation of PAH it was found that their concentrations are elevated during the winter months where heating systems are operating. On a daily basis it appears that the highest abundances of PAH occur on Wednesdays while the lowest on Sundays. The mean yearly concentration values of benzo[a]pyrene varied between 0.32±0.02 and 2.63±0.08 ng m −3 for the various sites. Compared with the reported values the PAH levels in the area of Ioannina can be characterized as medium to low.

The composition and sources of PM 2.5 organic aerosol in two urban areas of Chile

Fine particle (PM 2.5) samples were collected, using a charcoal diffusion denuder, in two urban areas of Chile, Santiago and Temuco, during the winter and spring season of 1998. Molecular markers of the organic aerosol were determined using GC/MS. Diagnostic ratios and molecular tracers were used to investigate the origin of carbonaceous aerosols. As main sources, road and non-road engine emissions in Santiago, and wood burning in Temuco were identified. Cluster analysis was used to compare the chemical characteristics of carbonaceous aerosols between the two urban environments. Distinct differences between Santiago and Temuco samples were observed. High concentrations of isoprenoid (30–69 ng m −3) and unresolved complex mixture (UCM) of hydrocarbons (839–1369 ng m −3) were found in Santiago. High concentrations of polynuclear aromatic hydrocarbons (751±304 ng m −3) and their oxygenated derivatives (4±2 ng m −3), and of n-alk-1-enes (16±13 ng m −3) were observed in Temuco.

Polynuclear Aromatic Hydrocarbon Concentration and Mutagenicity of Airborne Particles in Urban Air: Comparison of Tokyo Area and Santiago.

Five mutagenic and/or carcinogenic polynuclear aromatic hydrocarbons (PAHs), and Salmonella mutagenicity in airborne particles collected at three stations (industrial, commercial/residential, and agricultural areas) in Santiago, Chile were measured, and compared with the same measurements in the Tokyo metropolitan area. PAH concentrations showed daily variations in both cities. The concentrations of four-ring PAHs were higher in the Tokyo area than in Santiago, and the concentrations of five-or-more-ring PAHs were higher in Santiago than in the Tokyo area. Mutagenic activities also showed daily variations in both places, and were generally higher in Santiago. In the Tokyo area, the mutagenic activities were higher under conditions of TA98-S9 mix than under TA98+S9 mix while in Santiago, the activities were similar under the two mixes. There were good correlations between PAH concentrations and mutagenic activities in both areas. In Santiago, mutagenic activities and PAH concentrations were higher in the commercial/residential area and the industrial area than in the agricultural area.

Sources and Bioavailability of Polynuclear Aromatic Hydrocarbons in Galveston Bay, Texas

Oyster and sediment samples collected from six sites in Galveston Bay from 1986 to 1998 were analyzed for polynuclear aromatic hydrocarbons (PAHs). Total concentrations of parent PAHs in oysters ranged from 20 ng g−1 at one site to 9,242 ng g−1 at another and varied randomly with no clear trend over the 13 year period at any site. Concentrations of alkylated PAHs, which are indications of petroleum contamination, varied from 20 to 80,000 ng g−1 in oysters and were in higher abundance than the parent PAHs, indicating that one source of the PAH contaminants in Galveston Bay was petroleum and petroleum products. Four to six ring parent PAHs, which are indicative of combustion source , were higher than those of 2–3 ring parent PAHs, suggesting incomplete combustion generated PAHs was another source of PAHs into Galveston Bay. Concentrations of parent PAHs in sediments ranged from 57 to 670 ng g−1 and were much lower than those in oysters. Sediments from one site had a high PAH concentration of 5,800 ng g−1. Comparison of the compositions and concentrations of PAHs between sediment and oysters suggests that oysters preferentially bioaccumulate four to six ring PAHs. PAH composition in sediments suggests that the sources of PAH pollution in Galveston Bay were predominantly pyrogenic, while petroleum related PAHs were secondary contributions into the Bay.

Effect of petrochemical sludge concentrations on changes in mutagenic activity during soil bioremediation process

AbstractThe present study was performed to assess the effect of the petrochemical sludge application rate on the mutagenic activity (Ames test) of soil and the persistence of mutagenic activity during laboratory soil bioremediation process. Sludge‐soil systems were prepared at four different sludge application rates (1.25, 2.5, 5, and 10% w/w). Unamended soil was used as a control. Immediately following sludge application, in the absence or presence of S9, a linear correlation between sludge application rates and mutagenicity was found but differed significantly (p < 0.05) from the control system only at higher application rates (5 and 10% w/w). The direct mutagenicity of all systems decreases during the bioremediation process, and after a year of treatment only the 10% system induced a mutagenic response that was significantly different from the control system. On the other hand, an initial increase of the indirect mutagenicity was observed at all application rates. The time required for observing this increase was inversely proportional to the initial sludge concentration. After a year of treatment, the indirect mutagenicity of all sludge‐amended soils was not significantly different but was significantly different from the unamended soils. The persistence of the direct mutagenic activity of the sludge‐amended soils was related to the sludge concentration, whereas the indirect mutagenic persistence was related to the relationship between easily degradable hydrocarbons and polynuclear aromatic hydrocarbons concentration and independent from the initial application rate.

LANDFARMING IN A PAH-CONTAMINATED SOIL

The present work describes a coke oven soil treatability study by landfarming, conducted on-site in a field scale facility covering 100 m2. The soil contamination was mainly due to high concentrations of polynuclear aromatic hydrocarbons (PAHs) up to 1140 mg/Kg dry weight (ΣEPA). Along the treatment process the soil was characterised at the chemical, microbiological and ecotoxicological levels. After 3 months a reduction of 63% in total PAHs concentration was observed, being detected a more pronounced reduction for PAHs with 2, 3 and 4 rings (79%). Concomitantly, a change in the composition of the microbial population was observed with a significant increase in the PAHs degrading and total heterotrophic colonies. Concerning the ecotoxicity and genotoxicity data no effect was detected in the treated soil samples eluates.

EFFECT OF PETROCHEMICAL SLUDGE CONCENTRATIONS ON CHANGES IN MUTAGENIC ACTIVITY DURING SOIL BIOREMEDIATION PROCESS

The present study was performed to assess the effect of the petrochemical sludge application rate on the mutagenic activity (Ames test) of soil and the persistence of mutagenic activity during laboratory soil bioremediation process. Sludge-soil systems were prepared at four different sludge application rates (1.25, 2.5, 5, and 10% w/w). Unamended soil was used as a control. Immediately following sludge application, in the absence or presence of S9, a linear correlation between sludge application rates and mutagenicity was found but differed significantly (p < 0.05) from the control system only at higher application rates (5 and 10% w/w). The direct mutagenicity of all systems decreases during the bioremediation process, and after a year of treatment only the 10% system induced a mutagenic response that was significantly different from the control system. On the other hand, an initial increase of the indirect mutagenicity was observed at all application rates. The time required for observing this increase was inversely proportional to the initial sludge concentration. After a year of treatment, the indirect mutagenicity of all sludge-amended soils was not significantly different but was significantly different from the unamended soils. The persistence of the direct mutagenic activity of the sludge-amended soils was related to the sludge concentration, whereas the indirect mutagenic persistence was related to the relationship between easily degradable hydrocarbons and polynuclear aromatic hydrocarbons concentration and independent from the initial application rate.

Comparison of Sampling Techniques and Evaluation of Semipermeable Membrane Devices (SPMDs) for Monitoring Polynuclear Aromatic Hydrocarbons (PAHs) in Groundwater

Groundwater was sampled at the site of a former manufactured-gas plant using three conventional methods (bailing, low-flow, and bailing with filtering) and long-term in-situ samplers (semipermeable membrane devices [SPMDs]). Samples from each technique were analyzed for polynuclear aromatic hydrocarbons (PAHs) and total suspended solids (conventional methods only) to determine the effect of sampling procedure on PAH concentrations. In general, among the conventional techniques, bailing indicated the highest PAH concentrations, and bailing/filtering the lowest, with low-flow sampling in between. High-molecular-weight PAHs were most prevalent in bailed samples and most impacted by filtering, suggesting these species are particle-associated. SPMD results corroborated this finding. SPMDs performed well in this study; SPMD-derived PAH concentrations in the groundwater were similar to values obtained by the conventional techniques. Furthermore, they allowed detection of some PAHs not detected by conventional approaches in wells with low levels of contamination. An alternative method to the traditional equations for estimating aqueous contaminant concentra tions from SPMDs is proposed. This method may be more appropriate in groundwater where low flows prevail. Overall, this study suggests that SPMDs have high potential for use in groundwater monitoring, producing results consistent with conventional grab-sampling techniques, providing enhanced sensitivity integrated over extended time periods, and facilitating accurate determinations of truly dissolved PAH concentrations.

Characterization of firefighter exposures during fire overhaul.

Previous studies have characterized firefighter exposures during fire suppression. However, minimal information is available regarding firefighter exposures during overhaul, when firefighters look for hidden fire inside attics, ceilings, and walls, often without respiratory protection. A comprehensive air monitoring study was conducted to characterize City of Phoenix firefighter exposures during the overhaul phase of 25 structure fires. Personal samples were collected for aldehydes; benzene; toluene; ethyl benzene; xylene; hydrochloric acid; polynuclear aromatic hydrocarbons (PNA); respirable dust; and hydrogen cyanide (HCN). Gas analyzers were employed to continuously monitor carbon monoxide (CO), HCN, nitrogen dioxide (NO2), and sulfur dioxide (SO2). Area samples were collected for asbestos, metals (Cd, Cr, Pb), and total dust. During overhaul the following exceeded published ceiling values: acrolein (American Conference of Governmental Industrial Hygienists [ACGIH] 0.1 ppm) at 1 fire; CO (National Institute for Occupational Safety and Health [NIOSH] 200 ppm) at 5 fires; formaldehyde (NIOSH 0.1 ppm) at 22 fires; and glutaraldehyde (ACGIH 0.05 ppm) at 5 fires. In addition, the following exceeded published short-term exposure limit values: benzene (NIOSH 1 ppm) at two fires, NO2 (NIOSH 1 ppm) at two fires, and SO2 (ACGIH 5 ppm) at five fires. On an additive effects basis, PNA concentrations exceeded the NIOSH recommended exposure limits (0.1 mg/M3) for coal tar pitch volatiles at two fires. Maximum concentrations of other sampled substances were below their respective permissible exposure limits. Initial 10-min average CO concentrations did not predict concentrations of other products of combustion. The results indicate that firefighters should use respiratory protection during overhaul. In addition, these findings suggest that CO should not be used as an indicator gas for other contaminants found in this atmosphere.

Decreased Release of PAHs from Soils as a Result of Field Bioremediation

In this study, the relationship between the release characteristics of contaminants from soil and the biodegradability of the contaminants was evaluated. The soils used were from a large-scale field site. A batch rate of release procedure was used to quantify the release of polynuclear aromatic hydrocarbons (PAHs) from manufactured gas plant site soils before and during bioremediation in a field land treatment unit (LTU). This evaluation assessed the performance of the field-scale LTU with respect to changes in the soil PAH concentrations and to changes in chemical release parameters. The practical implications of the evaluation are discussed. During 147 days of LTU operations, the concentrations of specific PAHs were reduced 39–93%. ROR studies were conducted on LTU soil samples obtained at Days 0, 58, 72, and 147 of treatment. As a result of 147 days of treatment in the LTU, the fraction of a PAH released rapidly (F) had decreased for 8 of 10 PAHs, and the value of k2 (the first-order rate constant describing slow release) had decreased for 5 of 10 PAHs. There were no increases in F or k2 for any of the PAHs as a result of treatment in the field LTU. The results indicated that (1) only a fraction of the total concentration of the individual PAHs in the LTU samples was readily available to be released from the soil; (2) the remainder was released at a much slower rate; and (3) the mass of PAH released from the LTU soils to the aqueous phase decreased considerably. In addition, the fraction of the chemical released rapidly (F) for the Day 0 soil sample provided a reasonable prediction of the extent of PAH loss during 147 days of biological treatment in the field LTU.

Exposure of pacific herring to weathered crude oil: Assessing effects on ova

AbstractIn order to determine if exposure to Exxon Valdez oil would adversely affect progeny, reproductively mature Pacific herring were confined in water contaminated with weathered crude oil. Progeny were generally not affected by a 16‐d parental exposure to initial aqueous concentrations of ≤58 μg/L total polynuclear aromatic hydrocarbons (PAHs), yielding concentrations of up to 9.7 μg/g in ova. In contrast, previous research indicated that a 16‐d direct exposure of herring eggs to similarly weathered oil was detrimental to developing embryos at total initial PAH concentrations of 9 μg/L. Progeny of exposed fish could have been insulated from toxic effects for two reasons. First, as an apparent result of partitioning and metabolism in parental tissues, lower concentrations and less toxic PAHs were preferentially accumulated by ova (primarily naphthalenes; 84–92%). Second, peak exposure concentrations occurred before cell differentiation. The opposite was true for directly exposed eggs; the more toxic multi‐ring PAHs (e.g., phenanthrenes and chrysenes) and alkyl‐substituted homologues were accumulated, and internal concentrations increased during cell division, differentiation, and organ development. Thus, Pacific herring embryos are more critically sensitive to oil pollution than are gametes.

Recovery of Pink Salmon Spawning Areas after theExxon ValdezOil Spill

Up to 70% of wild pink salmon Oncorhynchus gorbuscha in Prince William Sound, Alaska, spawn in intertidal stream areas, many of which were contaminated by the 1989 Exxon Valdez oil spill. To assess recovery of salmon habitat after the spill, we analyzed sediment samples from stream deltas throughout Prince William Sound from 1989 to 1991 and 1995. In 1989, petroleum hydrocarbon concentration at 172 stream deltas (1–8 samples each) was bimodally distributed: 85 deltas had no detectable petroleum hydrocarbons (detection limit, 2 μg/g), whereas 87 deltas had a concentration of 2–90,000 μg/g. In 1995, petroleum hydrocarbons were still detected at eight of nine oiled streams that we resampled, with mean concentration up to 242 μg/g. The polynuclear aromatic hydrocarbon (PAH) fraction was also analyzed in 1995 to determine its origin, state of weathering, and toxic potential of the residual oil. The PAH fraction consisted mostly of the more toxic compounds with high molecular weight (e.g., phenanthrenes and chrysenes), and PAH composition was consistent with weathered Exxon Valdez oil. Total PAH concentration in 1995 averaged 0–1,300 ng/g, which was below the minimum sediment concentration (3,800 ng/g) shown to reduce salmon embryo survival in the laboratory. Interpolation between 1989 and 1995 indicated that residual PAH concentration exceeded 3,800 ng/g at some stream deltas through 1993, but all streams were below this level by 1994. We conclude that tidal leaching of PAH from weathered oil into incubation substrate could explain persistent elevated embryo mortality observed in pink salmon through 1993, and that spawning habitat had recovered to below lethal threshold by 1994.

Polynuclear Aromatic Hydrocarbons in American Oysters Crassostrea virginica from the Terminos Lagoon, Campeche, Mexico

The concentrations of polynuclear aromatic hydrocarbons in oyster tissues from one of the most productive shrimp-fishing and oyster-farming areas in the coastal Gulf of Mexico, the Terminos Lagoon, were measured to assess the impact that offshore crude oil production on the continental shelf of Campeche might have on local resources. Total PAH concentrations in oyster tissues ranged from 2470 to 42,500 ng/g during this six-month study. The predominance of low and medium molecular weight alkylated compounds over their parent compounds indicates the petrogenic source of these PAHs. The apparent correlation between the geographical distributions of the measured PAH concentrations and the distinctive circulation pattern into and inside the lagoon suggests that offshore oil activities are the source of these PAHs to local oysters. Although comparable, the concentrations reported in this study are on the high side of those reported earlier in samples from the same general area and from the northern coast of the Gulf of Mexico. This should be a cause for concern in a protected area where oysters are farmed for human consumption and should result in the implementation of monitoring programs.

A statistical approach for determining the environmental impact of polynuclear aromatic hydrocarbons in an oil spill-contaminated coastal area

Frequently, samples are collected and analyzed in order to investigate the extent of pollution in a soil or coastal zone. The resulting concentrations of pollutants at various sites are compared with the standard concentration levels in order to determine compliance with environmental regulations. These methods, however, have drawbacks when the area polluted is very large or has heterogeneous structures. In this study, a methodology is proposed which is based on the extent of pollution of polynuclear aromatic hydrocarbons (PAHs). As a model, a coastal area contaminated by an oil spill was selected and sediment samples were collected from the intertidal zone. Sixteen different PAHs were analyzed. The method involved a statistical approach, in order to obtain the probability density function, geometric standard deviation and the geometric mean for the collected data. In addition, toxicity equivalence factors (TEFs) for seven PAHs were used for estimating the pollution level. Based on the total PAH concentrations and the concentration of each PAH component, pollution probabilities above the standard pollution level set by the National Oceanic and Atmospheric Administration and the Florida Department of Environmental Regulation were calculated. The results also showed that acenaphthene and benzo(a)anthracene could be two representative components among 16 PAHs for the typical pollution index in the oil-spilled coastal region. The extent of pollution at sites could be expressed by the toxic concentration converted with the corresponding TEF of each PAH.

Histopathology of adult Pacific herring in Prince William Sound, Alaska, after the Exxon Valdez oil spill

Pacific herring (Clupea pallasi) sampled from oiled sites in Prince William Sound, Alaska, U.S.A., 3 weeks after the 1989 Exxon Valdez oil spill had multifocal hepatic necrosis and significantly increased tissue concentrations of polynuclear aromatic hydrocarbons (PAH). By comparison, Pacific herring from reference sites in 1989 and from all sites in 1990 and 1991 did not have hepatic necrosis or increased PAH concentrations. Adult Pacific herring were sampled for histopathology of liver, spleen, and kidney from oiled and reference sites in April (1989 and 1991) and October (1990 and 1991). Increased scores for macrophage aggregates contributed to significant differences in 1990, but these differences probably resulted from sampling older fish from the oiled site. Naphthalenes were the predominant PAH in all tissue samples. The development of hepatic necrosis and the predominance of naphthalenes in samples from 1989 is consistent with recent laboratory study in which crude oil exposure resulted in dose-dependent expression of viral hemorrhagic septicemia virus (VHSV). We conclude that Pacific herring were exposed to Exxon Valdez oil in 1989 and that development of hepatic necrosis in exposed fish probably was a result of VHSV expression.

Sensitivity of fish embryos to weathered crude oil: Part I. Low‐level exposure during incubation causes malformations, genetic damage, and mortality in larval pacific herring (Clupea pallasi)

AbstractPacific herring eggs were exposed for 16 d to weathered Alaska North Slope crude oil. Exposure to an initial aqueous concentration of 0.7 parts per billion (ppb) polynuclear aromatic hydrocarbons (PAHs) caused malformations, genetic damage, mortality, and decreased size and inhibited swimming. Total aqueous PAH concentrations as low as 0.4 ppb caused sublethal responses such as yolk sac edema and immaturity consistent with premature hatching. Responses to less weathered oil, which had relatively lower proportions of high molecular weight PAH, generally paralleled those of more weathered oil, but lowest observed effective concentrations (LOECs) were higher (9.1 ppb), demonstrating the importance of composition. The LOEC for more weathered oil (0.4 ppb) was similar to that observed in pink salmon (1.0 ppb), a species with a very different development rate; by inference, other species may be similarly sensitive to weathered oil. Our methods simulated conditions observed in Prince William Sound (PWS) following the Exxon Valdez oil (EVO) spill. Biological effects were identical to those observed in embryolarval herring from PWS in 1989 and support the conclusion that EVO caused significant damage to herring in PWS. Previous demonstration by our laboratory that most malformed or precocious larvae die corroborates the decreased larval production measured after the spill.

Sensitivity of fish embryos to weathered crude oil: Part II. Increased mortality of pink salmon (Oncorhynchus gorbuscha) embryos incubating downstream from weathered Exxon valdez crude oil

AbstractWe incubated pink salmon embryos under three exposure conditions, direct contact with oil‐coated gravel, effluent from oil‐coated gravel, and direct contact with gravel coated with very weathered oil (VWO). Embryo mortalities and polynuclear aromatic hydrocarbon (PAH) accumulation in embryo tissues during the direct‐contact and effluent exposure experiments were not significantly different, indicating that PAH accumulation was mediated by aqueous transport. Mortality rates for embryos exposed initially to a total PAH concentration (TPAH) of 1.0 ppb were significantly higher than controls when the PAH were derived from VWO. The same aqueous TPAH concentration failed to increase mortality rates when the PAH were derived from less weathered oil, indicating that toxicity of effluents from the VWO was primarily associated with the larger PAH. We conclude that water quality standards for TPAH above 1.0 ppb may fail to protect fish embryos. Further, pink salmon embryos incubating in Prince William Sound after the Exxon Valdez oil spill may have accumulated lethal concentrations of PAH from interstitial water that was contaminated when it percolated through oil reservoirs located upstream from salmon redds.

A currently forming petroleum associated with hydrothermal mineralization in a submarine caldera, Kagoshima Bay, Japan.

Petroleum-like products have been found in hydrothermally altered recent sediments of the submarine Wakamiko Caldera (depth = 200 m) in the northern Kagoshima Bay, southern Kyushu, Japan. High concentrations of polynuclear aromatic hydrocarbons (PAHs) of pyrolytic origin and biomarkers signatures representing diverse degrees of maturation have indicated that the associated bitumen is a typical hydrothermal petroleum. This is the first report on a significant content of hydrothermal petroleum related with a submarine volcanism of island-arc. The petroleum was generated by pyrolysis of organic matter in recent sediments deposited in the caldera. The hydrothermal petroleum and massive sulfide deposits are concluded to be currently forming within the caldera by hydrothermal activities below the seafloor where presently forming Kuroko-type sulfide deposits were also reported. Considering simultaneous formation of the hydrothermal petroleum and the Kuroko-type deposits in the Wakamiko Caldera, we propose a hypothesis that, during the middle Miocene, the same phenomena happened extensively in northeast Japan in association with the large-scale marine volcanisms, resulting in the formation of both petroleum and sulfide ore deposits (Kuroko) in the same geological horizons.