ZINC AND COPPER ACCUMULATION IN SEDIMENTS IMPACTED BY LANDFILL WASTEWATERS IN JARDIM GRAMACHO, SE BRAZIL

WT Machado^1*, MF Carvalho², M Moscatelli¹, LG Rezende¹, JEL Maddock¹, RE Santelli¹ and LD Lacerda¹

1- Dept. Geoquímica, Universidade Federal Fluminense, Niterói, RJ 24020-007, Brazil

2- PETROBRÁS, CENPES, DIVEX, CEGEQ, Rio de Janeiro, RJ 21949-900, Brazil

geowmac@vm.uff.br

ABSTRACT

We investigated the Zn and Cu accumulation in sediments affected by a large landfill facility. Metal distribution similarities in individual sediment cores suggest a common proximate source. Landfill leachate concentrations also presented a consistent relationship between metals. These results are consistent with a mixing of landfill leachate before its release to adjacent sites, resulting in a consistent Zn/Cu ratio. Metal relationship in more recent sediments from our sampling sites showed a degree of proximity to those from landfill leachate in the order river bank > degraded mangrove > mangrove forest. Sediments from the mangrove forest and degraded mangrove sites seem to present a higher retention efficiency for Zn than Cu. This suggest that a mangrove area located between the landfill and the river probably affected past metal transport from the landfill to river sediments, explaining the observed lower Zn/Cu ratio at the landfill-impacted area than in upstream sewage-impacted sediments.

INTRODUCTION

The contribution of different sources of heavy metals affecting impacted environments should be elucidated in order to recognize heavy metal pollution effects and to subsidy management strategies. Here, we present data on the identification of the contribution of a large 20-years-old landfill, located in a mangrove area in Jardim Gramacho (22°45’W, 43°15’S), at the west margin of Guanabara Bay (SE Brazil), to the Zn and Cu sediment load in the surrounding area. Guanabara Bay is the most important enclosed coastal bay in Brazil, and is characterized by anthropogenic pressures associated to urban and industrial development. With a domestic runoff discharge from at least 7.8 million of people, and the operation of about 6,000 industries around its margins, the bay’s environmental quality has become critical (Kjerfve et al., 1998). In a preliminary study, we evaluated the use of heavy metals as tracers of the past landfill contribution to the sediment metal load, to study the influence of landfill sites on contaminated areas and water courses.

MATERIAL & METHODS

Two long sediment cores (nearly 1 m length) were collected in a mangrove forest site (FM site), dominated by Laguncularia racemosa, and in a site where mangrove vegetation was degraded (DG site), between the landfill and Guanabara Bay. One short sediment core (nearly 30 cm length) was collected from the banks of the Sarapuí River (SR1 site), in the landfill landward edge. Two samples of surface layers (10 cm depth) from Sarapuí River bottom sediments were also collected, nearly 1 km upstream the landfill influence area (SR2 site), and seven landfill leachate samples were collected from interconnected leachate stabilization ponds (distanced at least 200 m), located along the landfill edges. Sediment profiles were sliced and dried in the laboratory. Mangrove roots were removed from the samples prior processing for analysis. Metal concentrations in sediments were analyzed by flame AAS and ICP-MS, after digestion in nitric and hydrochloric acid solutions. Leachate metal concentrations were analyzed by ICP-MS, after a microwave digestion (EPA method 3015). Precision, based on sample replicate analysis, was better than 9% for sediment samples and within 11% for leachate samples.

RESULTS & DISCUSSION

We found elevated Zn and Cu concentrations in upper sediment layers of all profiles (Table 1). Highest concentrations reached values as high as 417 mg Zn g^-1 and 120 mg Cu g^-1 at the site SR, 360 mg Zn g^-1 and 36 mg Cu g^-1 at the FM site, and 170 mg Zn g^-1 and 44 mg Cu g^-1 at the DG site. At a subsurface depth, which ranged from approximately 16 to 32 cm, cores from sites FM and DG showed a relatively coincident concentration increase for both elements, in relation to deeper layers. Above these depths of concentration increase, Zn and Cu were well correlated in cores from stations FM (P < 0.024) and DG (P < 0.001), as well as along the entire S1 profile (P = 0.001). Such similarities suggest a common proximate source for both elements.

The potential of heavy metals as tracers for contamination and sedimentation history has been demonstrated by the use of concentration-depth variability as a record of metal inputs (Caçador et al., 1996; Payne et al., 1997), if the sediment metal load is preserved without significant changes (e.g. by physical disturbance or diagenesis). Based on these assumptions, we hypothesized that the observed metal concentration increase in subsurface sediment layers were associated to landfill metal emissions, started in a period which corresponds to the depth of concentration increase in sediment cores.

Leachate metal concentrations ranged from 140-560 mg Zn L^-1 and from 70-170 mg Cu L^-1, while the duplicate sediment samples from SR2 site presented values of 332-356 mg Zn g^-1 and from 17-40 mg Cu g^-1. We speculate that past drainage runoff (diluting and transporting the leachate) and wastewater overflow from the landfill facility composed a major factor affecting the past metal input to all studied sites, whereas the upstream river sediments represent another metal source end member.

Another way of using heavy metals as tracers for the contamination record is the evaluation of the metal concentration-ratio variability as a fingerprint of the contamination source (Krumgalz, 1993; Benoit et al., 1999a,b). Besides the metal record preservation without significant changes with time, this evaluation is also based on the assumption that the source presents a unique and consistent ratio of metals. Leachate Zn/Cu ratio showed a relatively low variability, with an average of 3.1 ± 0.8 (± 1 SD), and both metals concentrations significantly correlated (P = 0.032). This suggests mixing of landfill leachate before its release to surrounding area, tending to present a relatively constant Zn/Cu ratio. Sediments from SR2 site showed fairly different metal ratios from those of leachate, ranging from 5.6-12, also indicating a high variability of metal associations in upstream sewage-affected sediments. Table 1 shows that metal concentration ratios tended to present values close to an average of 3.3, along core S1. Long sediment cores showed a trend of metal ratio increase from deeper to upper layers, except by relatively low variable values of core F3, implying changes in metal input, retention and/or post-depositional alterations. Metal-richer recent sediments from FM site averaged ratios of 9.4 (core F3) and 7.0 (core F4), while in station DG cores averaged ratios of 4.9 (core D1) and 4.7 (core D2).

These results suggest similar ratios between core S1 and landfill leachate, whereas cores from station DG showed a higher degree of proximity to the low leachate ratio, in contrast to values from deeper layers, and a lower degree of proximity to the leachate ratio was observed for FM site data. As demonstrated in Figure 1, considering the line of average leachate metal ratio as representing a metal source end member, according to the principle applied by Benoit et al. (1999a), we observed a clear degree of proximity to the landfill data in the order core S1 > core D1 > core D2 > core F4 > core F3.

Table 1. Zinc and copper concentration profiles (mg g^-1) and Zn/Cu ratios in sediment cores. Below 30 cm depth, average values are shown. na = not available.

	Core F3			Core F4			Core D1			Core D2			Core S1*
Depth (cm)	Zn Cu ratio			Zn Cu ratio			Zn Cu ratio			Zn Cu ratio			Zn Cu ratio
0–2	245	29	8.4	146	27	5.4	127	33	3.9	164	37	4.5	na	na	na
2–4	293	32	9.2	226	29	7.7	113	25	4.6	171	37	4.6	417	113	3.7
4–6	225	29	7.8	221	35	6.3	132	31	4.2	157	36	4.4	367	106	3.5
6–8	230	28	8.3	246	36	6.8	137	35	3.9	160	39	4.1	332	101	3.3
8–10	361	30	12	259	36	7.3	143	35	4.1	162	41	4.0	287	88	3.3
10–12	209	26	8.2	236	31	7.5	115	25	4.6	174	44	4.0	267	83	3.2
12–14	256	27	9.6	178	26	6.8	93	22	4.3	165	43	3.9	283	81	3.5
14–16	185	20	9.1	121	15	8.0	142	28	5.0	154	28	5.4	276	79	3.5
16–18	174	15	12	67	8.2	8.2	174	30	5.9	156	38	4.1	239	76	3.1
18–20	78	8.0	9.7	52	5.6	9.2	157	28	5.6	160	31	5.1	296	96	3.1
20–22	65	4.8	13	58	7.2	8.1	120	25	4.7	153	28	5.5	352	121	2.9
22–24	48	6.5	7.4	65	6.8	9.6	102	18	5.8	146	26	5.7	310	106	2.9
24–26	75	7.4	10	77	12	6.7	107	19	5.7	124	21	5.9
26–28	90	7.3	12	92	8.2	11	102	18	5.7	88	15	5.9
28–30	73	7.5	9.7	59	6.4	9.2	99	17	5.7	92	14	6.7
> 30	56	5.7	10	59	5.9	11	53	7.7	7.1	65	9.3	7.4

* For core SR1, the depth intervals indicated as 20–22 cm and 22–24 cm correspond to depths 20–25 cm and 25–30 cm, respectively.

Sediments from SR2 site showed similar Zn concentrations and much lower Cu concentration than SR1 site sediments, suggesting that upstream sediments are not sufficiently Cu-rich to promote the low Zn/Cu ratio observed in SR1 site. If the trends above can be generalized for mangrove sediments, the mangrove area located between the landfill and the river probably affected the past metal transport to SR1 site sediments, explaining the lower Zn/Cu ratios at the landfill-impacted than in upstream sediments. The lack of similarity of metal ratios, “different fingerprints”, may be an indicator of metal redistribution (Krumgalz, 1993). Cores from sites FM and DG showed higher Zn/Cu ratios in upper layers than landfill leachate, possibly due to diagenetic processes in mangrove rhizospheres, resulting in a higher Zn retention capacity than for Cu, and/or due to a contribution of other sources. In fact, all cores consisted of fine-grained and gray to black colored sediments, except by subsurface orange-gray mottled horizons presented by mangrove forest sediments, due to sediment oxidation by O₂ release by mangrove roots, inducing differentiated geochemical conditions (Lacerda, 1998). Moreover, we have observed a lack of correlation between Zn and Cu distributions in another mangrove forest site in Jardim Gramacho (Machado, unpubl. data), associated to high Zn/Cu ratios.

Modifications in the relationship of wastewater constituents can occur with the time, during dilution and transport processes and after accumulation in a new environment, and should be better evaluated in order to validate the application of our preliminary approaches. However, the results above attest the importance of the landfill contribution to Zn and Cu accumulation at the studied sites, and can explain the different trends of metal relationship observed.

Figure 1. Relationship between Cu and Zn concentrations in landfill leachate ´ 200 (Û) and in metal-rich surface and subsurface layers of sediment cores from FM site (s, core F3; ¿, core F4), DG site (¯, core D1; r, core D2) and SR1 site (+, core S1). The line indicates the average metal ratio of landfill leachate samples.

ACKNOWLEDGEMENTS

We acknowledge the logistic support from Queiroz Galvão Engenharia, the analytical support from PETROBRÁS/CENPES and grants to WM, LGR, RES and LDL from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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