Distribution and Normalization of Heavy Metal Concentrations in Mangrove and Lagoonal Sediments from Mazatlán Harbor (SE Gulf of California)

Estuarine, Coastal and Shelf Science (2001) 53, 259–274 doi:10.1006/ecss.2000.0814, available online at http://www.idealibrary.com on

Distribution and Normalization of Heavy Metal Concentrations in Mangrove and Lagoonal Sediments from Mazatla´n Harbor (SE Gulf of California) M. F. Soto-Jime´neza and F. Pa´ez-Osunab,c a

Programa de Posgrado en Ciencias del Mar y Limnologı´a, Estacio´n Mazatla´n, UNAM, Me´xico Unidad Acade´mica Mazatla´n, Instituto de Ciencias del Mar y Limnologı´a, UNAM, Apdo. Postal 811, Mazatla´n 82000, Sinaloa, Me´xico

b

Received 19 October 1999 and accepted in revised form 15 November 2000 Concentrations of heavy metals, carbonates, organic carbon and granulometry were examined in sediments from 60 sites within Mazatla´n Harbor and adjacent areas. Regional distribution had a strong (for Al, Fe, Li and Ni) and weak (for Cd, Co, Cr, Pb, V and Zn) seaward concentration gradient decreasing from the upper lagoon. The highest concentrations for most metals occurred in fine-grained sediments from Infiernillo Estuary, the upper lagoon and the industrial zone. In contrast, lower levels were usually found in the sandy sediments of the navigation channel, port entrance and an area associated with sewage outfall. Analysis of transects in mangrove and lagoonal sediments indicated that the amount of fine material and organic carbon increases towards the margins where mangrove sediments exist. While metal variations were not clearly observed in most of the metals examined; only Ni, V, Pb and Cu showed a slight tendency to increase towards the margins. Sometimes lagoonal sediments had redox and texture characteristics comparable to those from mangrove substrate, thus competing because of a similar capture capacity of metals. Metal data were normalized against Al and Li using a combination of normalization techniques (95% prediction intervals, regional anomalies and enrichment factor). It was found that Al and Li were good normalizers for most of the examined metals and they are important constituents of one or more of the major fine-grained heavy metal carrier(s) and adequately reflect the granulometric variability in the sediments of the study area.  2001 Academic Press Keywords: heavy metals; sediments; lagoon; mangroves; normalization; Gulf of California

Introduction In Mexico approximately 125 coastal ecosystems have been classified as coastal lagoons with respect to geological characteristics (Lankford, 1977). Many of the drainage basins of these ecosystems are experiencing the effects of industrial, aquacultural and agricultural expansion, and an accelerated population growth. In addition, a significant part of this population depends on lagoon resources for stable dietary items and as a tourist attraction. Coastal lagoons include a great variety of habitats: mangrove forests, salt marshes, swamps, freshwater inner lagoons, tidal channels, brackish and sea water systems. The influence of anthropogenic input of metals and other contaminants is complex in these habitats, and could result in changes in their ecological stability. Elevated metal concentrations related with longterm pollution caused by human activities have been recorded in mangrove sediments (Lacerda et al., 1993; Mackey et al., 1992; Mackey & Hodgkinson, 1995; c

Corresponding author. E-mail: [email protected]

0272–7714/01/90259+16 $35.00/0

Tam & Yao, 1998). Similarly, several studies (e.g. Harbison, 1986; Lacerda et al., 1993; Tam & Wong, 1993, 1995) have shown that mangrove sediments have a high capacity to retain heavy metals from tidal water and stormwater runoff, and therefore they often act as sinks for heavy metals. Since natural and anthropogenic material accumulates simultaneously in sediments, it is difficult to identify the proportion of each source. The problem is exacerbated by the magnitude of the variation in the loading of elements originating from these two sources, as well as the interaction of elements with sediment grain size, mineralogy and organic carbon content. Several granulometric and geochemical procedures have been developed to compensate for such influence. One procedure involves normalization of metal data to the different size fractions of sediment (e.g. Luoma & Bryan, 1981; Horowitz et al., 1990; Szefer et al., 1995). However, the obtained information is incomplete because the procedure requires separation steps and therefore does not reveal the bulk element composition of the sediment.  2001 Academic Press

260 M. Soto-Jime´ nez and F. Pa´ ez-Osuna

Geochemical procedures, on the other hand, include normalization in relation to a conservative element such as Al (Bruland et al., 1974; Martin & Meybeck, 1979; Windom et al., 1989), Fe (Szefer, 1990; Herut et al., 1993; Tam & Yao, 1998), Rb (Grant & Middleton, 1990) and Li (Loring, 1990). A normalization procedure is based on the fact that the proportions of a metal in relation to a conservative element are relatively constant in the crust (Schropp et al., 1990); these proportions have been demonstrated in estuarine and coastal sediments (e.g. Windom et al., 1989; Din, 1992; Covelli & Fontolan, 1997). However, little is known about the suitability of this procedure in mangrove and lagoonal sediments. From this fact, the objective of this study was to examine the relationships of metal concentrations to Al and Li concentrations in order to determine the level of metal contamination in Mazatla´ n Harbor and adjacent areas and to detect any anthropogenic metal contribution to the lagoon system. Additionally, the composition of mangrove and lagoonal sediments in the upper lagoon region was examined to determine relationships between granulometry, organic carbon and metal contents. Study area The study area is located along the southeast coast of the Gulf of California, and includes the lagoon system known as ‘ Estero de Urı´as ’ and the adjacent coastal area. Mazatla´ n Harbor is situated in the lower and intermediate part of the coastal lagoon system (Figure 1). The lagoon has an area of c. 16 km2 with an orientation axes semi-parallel to the coast. The bathymetry is predominantly shallow (0–2 m) with the exception of the erosional and artificial channels in the upper lagoon (2–4 m), and navigation area (5–12 m). Lankford (1977) classified this system as a coastal lagoon with an inner barrier shelf. According to Pritchard (1967) this water body can be considered as an estuarine system during the rainy season and an anti-estuarine system in the dry season. The climate of the region is tropical subhumid with a monthly average temperature ranging from 19·7 C in February to 28·0 C in August. Annual average precipitation is about 800 mm, occurring mainly during the rainy season from July to October (Garcı´a, 1973). The lagoon is predominantly vertically mixed (salinity range 25·8–38·4) with an average tidal amplitude of 1·5 m and water velocities of 0–0·50 m s
1. Therefore the residence time of the water is relatively short (Pa´ ez-Osuna et al., 1990) and the industrial and domestic effluents can be expelled thoroughly. However, in calm areas significant amounts of

contaminants can accumulate and thus represent a potential hazard for the system (Ruelas-Inzunza & Pa´ ez-Osuna, 1998). The lagoon system receives major freshwater inputs from land runoff and small streams located in the Upper Lagoon (UL) that includes a group of tidal channels (1–4 m deep) where the mangroves Rhizophora mangle, Laguncularia racemosa and Avicennia germinans are present. The UL receives discharges of 2·2106 m2 of semi-intensive shrimp farm ponds. The Head Lagoon (HL) close to UL is a wide (300–1200 m) and shallow (1–2·5 m deep) area also characterized by the presence of mangroves along the margins, but without direct anthropogenic discharges. The Industrial Zone (IZ) is located in the middle and lower parts of the lagoon system (500–2000 m); mangroves are present only in the southern part. This region is subject to regular discharges of effluents from the fish and shrimp processing industry, the sandblasting of boats and the thermoelectric plant. The Navigation Channel (NC) has a bathymetry of up to 12 m, mangroves are absent, and the western margin includes the port installations where the navy, fishing and merchant fleets are found. The Port Entrance (PE) is in the coastal area adjacent to the mouth (10–15 m deep) of the lagoon system. The Sewage Outfall (SO), includes a marine area (8–16 m deep) around the municipal sewage treatment plant that receives 1500–2000 l s
1 of raw sewage and has a capacity for primary treatment of only 650 l s
1, therefore untreated and partially treated wastewater is discharged into this region. The study area also receives untreated domestic effluents (37 800 m3d
1) from c. 30% of Mazatla´ n population (Osuna-Lo´ pez et al., 1997) in a small water body known as the Infiernillo Estuary (IE). This area is small (4 105 m2) and shallow (1–2 m deep) and is connected with the main lagoon via a narrow channel and is characterized by the presence of mangroves (R. mangle) in the inner portion. Materials and methods Sampling Surface sediments were sampled with a 0·3 m2 Van Veen grab at 60 stations from August to November 1994 (Figure 1). Representative stations were selected in seven regions of lagoon system, which was divided considering the type of human activities, bathymetry, morphology, and the degree of marine influence. These regions were Infiernillo Estuary (EI), Upper Lagoon (UL), Head Lagoon (HL), Industrial Zone (IZ), Navigation Channel (NC), Port Entrance (PE)

Distribution and normalization of heavy metal concentrations 261

4 5 6

E1

7

LAN ZAT MA

13'

11'

Y CIT

N LA AT Y AZ A M B SO

30 29 31 32 33

28 27 26 25

IZ

34

y tu ar es es 10

22

18

20

21

Shrimp Farm

9

11

17 Ba

12 13

35

rro

ne

UL

36 16 37

stu

ar

y

15

NC

PACIFIC OCEAN

38 49 48 47 46 45

23

8

19

HL 24

Co nf it

Infiernillo estuary 1 2 3

ico ex

15'

M

Thermoelectric Industrial plant Urban area Mangroves

39 41 42 43 40

EP

N

0

1

2

3 km

44

25'

23'

21'

A 2

3

1

4 5

2

6

3

7 8 6

C

5

4

7 B

8

0.25 km

F 1. Sampling stations in Mazatla´ n Harbor and the following regions: Infiernillo Estuary (EI), Upper Lagoon (UL), Head Lagoon (HL), Industrial Zone (IZ), Navigation Channel (NC), Port Entrance (PE) and Sewage Outfall (SO). Transects A-B and A-C are shown as inset of the UL region.

and Sewage Outfall (SO). Each sediment sample was carefully taken from the central portion of the dredge with a plastic spatula previously washed with 2M HCl and 2M HNO3 (Moody & Lindstrom, 1977). Samples were stored in plastic bags at 4 C and frozen at
20 C after transport to the laboratory. Representative portions of the sediments were oven-dried at 70 C for 5–7 days, ground to fine powder using a porcelain mortar and pestle. Each ground sample was then stored in clean plastic bags prior to analyses. Analyses Granulometric analysis was carried out using standard sieve and pipette techniques after organic matter

destruction with H2O2 (Covelli & Fontolan, 1997). Sediment texture and statistical grain size parameters were calculated according to Folk (1974). The organic carbon content was determined by oxidation with 1N K2Cr2O7 acidified with concentrated H2SO4 and titration with 0·5 N Fe (NH4)2(SO4)2 (Loring & Rantala, 1992). The carbonate content was determined by reaction with 1 M HCl, and the excess of acid was determined by back-titration with 0·5 M NaOH using phenolftaleine as an indicator (Rauret et al., 1988). Variation coefficients of replicate analysis (n=6) ranged from 0·4 to 6·3% for carbonate, and 0·5 to 4·8% for organic carbon. The analytical procedure for metal determination was that described by Pa´ ez-Osuna and Osuna-Lo´ pez

262 M. Soto-Jime´ nez and F. Pa´ ez-Osuna

(1990). Samples and blanks were leached at about 130 C with a 3:1 mixture of concentrated HNO3 (17M) and HCl (12M) (Breder, 1982) in a Teflon decomposition manifold system. After digestion, residues were centrifuged (3500 rpm) and diluted solutions were analysed by flame atomic absorption spectroscopy using a Shimadzu AA-630. All plastic material and glassware were thoroughly washed with 2 M HCl and 2 M HNO3. Quality control was provided by analytical checks on blanks and by an intercalibration international Atomic Energy Agency sediment standard SD-N-1/2 (IAEA, 1985). Metal recovery was 15% of certified values, with the exception of Li and Pb. Li exhibited a recovery of 65%, but during the intercalibration results, the confidence interval established for Li was not certified (IAEA, 1985). Pb showed a recovery of 128–141% respect to the confidence interval (
=0·05) given. However, such estimations do not modify the discussion of this investigation. Analytical precision of replicates (n=6) of three different samples varied from <12% relative standard Deviation (RSD) for Cd, Cr, Cu, Co, Mn, Ni, Li, Pb, V and Zn to <21% for Fe and Al. The detection limits were Al, 2·4; Cd, 0·3; Co, Cr, Li and Zn, 0·5; Cu, Fe and Ni, 1·0; Pb, 2·6; and V, 3·2 mg kg
1. Data processing An evaluation of the correlation between normalizing elements (Al and Li) and metals using regression analysis was made with the entire data points. Data were examined for statistical outliers. Points outside the 95% prediction limits were removed to avoid the inclusion of any sample receiving anthropogenic sources of metal in the final regression model and to ascertain that statistical relationships were solely based on natural concentrations (Loring, 1990; Din, 1992; Summers et al., 1996). Additionally, sediment quality guidelines developed by Long and Morgan (1990) were used to discard metal values at which adverse biological effects may occur (Summers et al., 1996). The concentration of metals considered toxic to biota obtained from a biological effects database (Long & Morgan, 1990; Long et al., 1995) are referred to as effects range-low (ERL) according to guidelines (Cd 1·2, Cr 81, Cu 34, Ni 20·9, Pb 46·7, and Zn 150 mg·kg
1). The possibility that unnaturally metal enriched samples were included in the final data set was minimized removing data points upper 95% prediction limits (Schropp et al., 1990) and data points with metal concentration greater than ERL (Summers et al., 1996). The number of data

points deleted using the normalization procedure were: V (16), Fe (14), Cd, Co, Cr and Cu (13), Ni, Pb and Zn (11). Concentrations >ERL were deleted four values for Cu and Pb, and two values for Ni. In the case of Ni and Pb, some data >ERL were retained because this way they produce a higher coefficient of regression. After all statistical outliers and values exceeding ERL guidelines were eliminated, regression models between each metal to Al and Li were repeated considering a ‘ natural ’ relationship which includes data points from sediments with null or very low probability of anthropogenic enrichment. Appropriate transformations to correct for nonconstant variance and non-normality were applied for some metals to ensure the assumptions used in linear regression analysis (Zar, 1984; Summers et al., 1996). For most metals, single or double square root transformation was sufficient to correct for heteroscedasticity and achieve an approximate normality (Schropp et al., 1990; Din, 1992; Summers et al., 1996). For each metal, a predicted value and standard error (upper 95% confidence limit for ‘ natural ’ sediments) were calculated in every sample by the regression equations using Al and Li content. These values were used to obtain a regional reference range for all points on the regression line for each metal, which is defined as the predicted value plus twice the standard error of estimate (12 s). The points lying outside the upper limit were easily discernible on the graphics; these sediment samples were potentially influenced by anthropogenic sources (Szefer et al., 1996; Covelli & Fontolan, 1997). For a better estimation of anthropogenic input, an enrichment factor was calculated for each metal by dividing its ratio to the normalizing element by the same ratio found in the chosen baseline. The enrichment factors (EFs) for each element were calculated from the formulae (Salomons & Forstner, 1984; Sinex & Wright, 1988; Grant & Middleton, 1990): EF=(M/Al or Li)sample/(M/Al or Li)crust The values for the earth crust are from Martin and Meybeck (1979) and represent the average composition of the surficial rocks exposed to weathering. This values are: Al 6·9%, Fe 3·6%, Cd 0·2, Co, 13, Cr 71, Cu 32, Li 42, Mn 720, Ni 49, Pb 16, V 97 and Zn 127 mg/kg. EFs around 1·0 indicate that the element in the sediment is originated predominantly from lithogenous material, whereas EFs much greater than 1·0 indicate that the element is of anthropogenic origin (Szefer et al., 1996).

Distribution and normalization of heavy metal concentrations 263 0

3

6

9

12 15 18 21 24 27 30 33 36 39 42 45 48 33

36

10

45

Clay (%)

75 33

50

0

36

25 45

39

30

COrganic (%)

15 15

0 3

10 5 0

9

22

5

34

6 3

22

30

Al (%)

4

–1

CaCO3 (%)

0

Li (mg kg )

Mz

5

0 29

34

45

20 10 0

EI

UL

HL

IZ

NC

EP

SO

F 2. Distribution of Mz, clay, organic carbon, carbonates, Al and Li in sediments collected from Mazatla´ n Harbor and adjacent areas. The numbers represent stations from Figure 1 and the line indicates direction of progressive sampling. The solid line represents the regression line that shows the global tendency of distribution.

Results Texture, organic carbon and carbonates Sediments from EI and UL consisted of silty clays with high organic carbon concentrations (1·4–11·4%) and relatively low content of carbonates (1·0–5·9%). HL consisted predominantly of sands with moderate organic carbon (1·1–4·1%) and carbonates (1·7– 7·8%) contents. The Industrial Zone was characterized by the predominance of sands with variable organic carbon (0·6–5·2%) and carbonate (1·3– 16·9%) contents. The sedimentary material in NC region consisted of sands with low organic content (0·9–1·7%) and variable carbonate content (4·8– 39·0%). Only in station 36, located in front of the tuna processing plant and the shipyards, showed siltyclays and a more elevated level of organic carbon (4·3%). Sediments from PE comprised sands and silty-sands with reduced organic carbon (0·6%–1·5%) and a carbonate content that ranged from 3·1 to 14·8%. Similarly, sediment from the SO region consisted predominantly of sands with moderate organic content (1·1–1·7% and relatively high content of

carbonates (8·6–23·6%). Station 45, close to the sewage outfall end, consisted of clayey silts with 2·1% of organic carbon. On a regional basis, the sequence of grain-size parameter (Mz) was EI>UL>HL>IZ>NC>SO>PE. The high percentages of fine fraction in sediments from EI and UL were due to the deposition of alluvium [soils of the Quaternary (DGG, 1983)] which gradually declined from the upper lagoon to the mouth (Figure 2). The prevalent hydrodynamics and the presence of mangrove roots, which are well developed in both EI and UL, retained this fine fraction. The characteristic root system of mangroves has a dense grid of vertical pneumatophores and aerial roots. This structure traps floating detritus and reduces tidal flow, eventually creating conditions where suspended clay and silt particles settle. This material is captured by a tangled mat of root hairs growing below the mud surface, hence stabilizing the mangrove substrate, which is rich in organic matter. Some stations, such as 33 and 36, had clay content relatively elevated in relation to the regional tendency. Station 33 is located on a shallow area where

264 M. Soto-Jime´ nez and F. Pa´ ez-Osuna

abundant macroalgae communities (Ulva lactuca, Enteromorpha intestinalis and Gracilaria sjoestedtii) occur and station 36 is situated in the navigation channel in front of the piers and shipyards. Carbonates ranged from 1·0% to 39·0%, with the highest value in station 39 (in the NC region) where abundant quantities of shell fragments were found. Distribution of carbonates in sediments gradually increased seaward, where favourable conditions (temperature, salinity, substrate, and presence of siliciclastics) for the growth of organism producers of calcium carbonate exist (Lees, 1975). The overall variation of organic carbon content was between 0·6 and 11·4% and tended to increase with the amount of fine-grained material. Regional metal distribution The patterns of spatial distribution for conservative elements (Al and Li) are shown in Figure 2. EI and UL regions had the highest values of Al and Li that coincide with high contents of silt and clay fractions and organic carbon. Concentrations showed a regional tendency for a gradual reduction from the upper lagoon to the mouth, which indicates that both metals follow the same pattern as the fine materials. Co, Cd, Cu, Cr, Fe, Ni, Pb, V and Zn showed a similar tendency, i.e. a decrease towards the mouth (Figure 3). The highest concentrations of these metals occurred in fine-grained sediments of EI, UL and IZ regions, and the lower levels were usually found in the sandy sediments of the adjacent region to the mouth (PE and SO). Co ranged globally 4·7 to 17·6 mg kg
1, a similar value to the average crust level. Cd concentrations were very similar in the seven regions, being 4–5 times higher than the background level suggested for surface rocks. Depending on the region and textural differences of sediments, Cu concentrations ranged from 7·7 and 90·9 mg kg
1. In comparison with value for earth crust, many stations had higher Cu levels. The tendency from the upper lagoon to the mouth showed a better uniformity of Cr levels, ranging from 7·6 to 42·5 mg kg
1. Concentrations were smaller than level considered as background (71 mg kg
1). Fe concentrations ranged from 1·4 to 9·5%; average levels in region EI were significantly higher than in regions UL, IZ, and NC. Ni levels ranged from 6·1 to 30·3 mg kg
1. Concentrations from IE were higher than in other areas, though the maximum value was low in comparison to a background level of 49 mg kg
1 in surface rocks. Pb concentrations showed certain uniformity with distance from the upper lagoon to the mouth and apparently almost all stations had higher

levels than those reported for natural systems. V concentrations ranged from 14 to 100 mg kg
1, the maximum value was comparable to the background level. Zinc ranged from 46·4 to 347·8 mg kg
1 with the same tendency to decrease towards the mouth. Some stations from EI and IZ showed high Zn concentrations than background level. The spatial distribution of Mn was different from that of the rest of the metals (Figure 3). Sediments from EI and UL had relatively low levels of Mn, with increasing levels towards the mouth of the lagoon system. Co-associations and normalization of metal concentrations The correlation matrix (Table 1) of organic matter that act as a metal carrier showed a poor positive correlation for most of the metals, only for Co, Cu, Fe and Ni concentrations was significant (P<0·05). The good correlation (P<0·001) between reference elements (Al and Li) and grain-size parameter (Mz vs Al, r=0·55; Mz vs Li, r=0·59; Al vs Li, r=0.84) indicated that both elements were important constituents of one or more of the major fine-grained trace metal carrier(s) and reflect a granular variability in the sediments (Loring & Rantala, 1992). Additionally, metals had a highly significant correlation with Mz. These granulometric relations showed that correlation occurred consistently when increasing metal concentrations and decreasing grain size. It is obvious that the coarsergrained material had a dilution effect on most metal concentrations, as supported by the significant (P<0·001) negative correlation between sand and metal contents. In the case of carbonate content there was no significant linear association with most metals, except for Mn which showed a weak positive correlation (r=0·29, P<0·05). Most of the metals examined in sediments from Mazatla´ n Harbor and adjacent areas exhibited a significant linear correlation with Al and Li; r-values ranged between 0·53–0·83 and 0·51– 0·83, respectively. The trend for Cr, Fe, V and Pb were similar to other metals, but with a slightly less significant linear correlation (r<0·58). It was evident that Mn did not covary with the reference elements (Al and Li) or any other metal, except for Co (r=0·29), P<0·05). Similar results have been reported for estuarine and coastal sediments (e.g. Windom et al., 1989; Din, 1992; Szefer et al., 1996). Results of regression analyses between each metal and normalizing elements are shown in Tables 2 and 3. It can be observed that for V and Fe only one axis was transformed, for Co only the metal concentration (y) was square-root transformed, and for the remaining metals, the metal and Al or Li

Distribution and normalization of heavy metal concentrations 265 0

3

6

9

12 15 18 21 24 27 30 33 36 39 42 45 48

20

–1

2

0 29 34

4

38

1 120 80

–1

60 40

0 4

43

5

20 0

12 5

40

8 45

–1

0

30 15 0

20

45

600

–1

150

0

36

–1

Pb (mg kg )

300

Mn (mg kg )

Ni (mg kg )

4

Fe (%)

4

100 50 150 49

5

100

450

0

300

29

–1

50

V (mg kg )

0 4

–1

–1

Cr (mg kg )

40

Cu (mg kg )

0 29 34

Zn (mg kg )

–1

Cd (mg kg )

10

Co (mg kg )

5

34

250 0

EI

UL

HL

IZ

NC

EP

SO

F 3. Distribution of Cd, Co, Cr, Cu, Fe, Ni, Mn, Pb, V and Zn in sediments collected from Mazatla´ n Harbor and adjacent areas. Numbers represent stations from Figure 1 and the connecting line indicates direction of progressive sampling. The solid line represents the regression line that shows the global tendency of distribution.

concentrations (y and x) were square-root transformed. Figures 4 and 5 show data as black points with solid lines representing the least squares regressions, and the recalculated equation as a broken line. Except for Mn and in a lesser extent Fe, these regressions give a much better fit for the metal concentrations, which suggests that a simple regression

model describe the natural behaviour of the metals in the study. Coefficient of determination (R2) values for the models ranged from 0·52 to 0·84 for Li and from 0·39 to 0·91 for Al. In the case of Fe and V, only 38% and 39% of the variation, respectively, could be explained by the use of regression models, due to a high degree of scatter in both sets of data.

1·00
0·38 0·77
0·91 0·73 0·91 0·88 0·39 0·37 0·44 — 0·39 0·31 0·48
0·30 0·38 0·45 — —

1·00
0·30 0·34 —
0·37
0·39 — — — — — — — 0·29 — — — —

CaCO3

1·00
0·64 0·53 0·63 0·61 — — 0·35 — 0·31 0·32 —
0·34 0·297 — — —

Org C

1·00
0·88
0·94
0·97
0·58
0·46
0·57
0·34
0·56
0·46
0·62 —
0·55
0·56
0·30 0·42

Sands

1·00 0·67 0·81 0·55 0·48 0·59 0·45 0·58 0·49 0·59 — 0·53 0·52 0·31 0·48

Silts

1·00 0·95 0·52 0·38 0·47 — 0·47 0·37 0·55 — 0·49 0·50 — 0·32

Clay

1·00 0·55 0·42 0·49 0·33 0·55 0·49 0·59 — 0·49 0·52 0·34 0·38

Mz

1·00 0·63 0·78 0·58 0·79 0·54 0·84 — 0·83 0·53 0·59 0·74

Al

0·00 0·75 0·58 0·78 0·44 0·69 — 0·78 0·64 0·57 0·74

Cd

Correlation coefficient significant r0·05(2),47 =0·282; r0·01(2),47 =0·365; r0·001(2),47 =0·456 (Zar, 1984). Not significant: —.

Moisture %CaCO3 Org C Sands Silts Clay Mz Al Cd Cu Cr Co Fe Li Mn Ni Pb V Zn

Moisture

1·00 0·56 0·69 0·42 0·76 — 0·83 0·68 0·46 0·92

Cu

1·00 0·74 0·66 0·51 — 0·60 0·48 0·68 0·54

Cr

1·00 0·76 0·83 0·29 0·82 0·55 0·81 0·67

Co

1·00 0·53 — 0·51 — 0·80 0·34

Fe

1·00 — 0·81 0·57 0·53 0·74

Li

1·00 — — — —

Mn

1·00 0·58 0·62 0·72

Ni

1·00 0·28 0·66

Pb

1·00 0·43

V

1·00

Zn

T 1. Correlation coefficient matrix showing the relationships between concentrations of heavy metals, organic carbon, and other parameters in sediments from Mazatlan Harbor and adjacent areas

266 M. Soto-Jime´ nez and F. Pa´ ez-Osuna

Distribution and normalization of heavy metal concentrations 267 T 2. Transformation and results of regressions that were applied to Al versus metal data. Correlation between metals (except Mn) and Al were significant (P<0·001) Metal Cd Cr Co Cu Fe Pb Ni V Zn

Transformation √y, √y, √y, √y, y, √y, √y, y, √y,

√x √x √x √x x √x √x x √x

N

R2

SD

Slope

Intercept

47 47 47 47 46 49 49 44 49

0·7 0·49 0·82 0·88 0·19 0·76 0·91 0·39 0·79

0·08 0·41 1·18 0·5 0·45 0·55 0·23 8·49 0·89

0·248 0·953 1·348 2·721 0·292 1·972 1·444 4·805 3·616

0·483 2·245 4·26 0·336 2·083 2·517 1·207 23·88 4·231

R2 =coefficient of determination. SD=standard deviation.

T 3. Transformation and results of regressions that were applied to Li versus metal data. Correlation between metals (except Mn) and Li were significant (P<0·001) Metal Cd Cr Co Cu Fe Pb Ni V Zn

Transformation

N

R2

SD

Slope

Intercept

√y, √x √y, √x √y, x √y, √x y, x √y, √x √y, √x y, x √y, √x

47 47 47 47 46 49 49 44 49

0·78 0·52 0·84 0·76 0·45 0·84 0·83 0·61 0·77

0·07 0·39 0·96 0·54 0·72 0·46 0·57 6·78 0·97

0·146 0·452 0·328 1·12 0·098 1·201 0·72 1·26 1·998

0·318 2·003 3·233 0·128 1·279 1·093 0·703 16·76 2·167

R2 =coefficient of determination. SD=standard deviation.

Statistically, slope coefficients for all the pairs of studied elements were positive and significantly different from zero (P<0·001) (Table 2 and 3). An intercept different from zero (>0·0) indicates that there are other sources of these elements in addition to aluminosilicates. Some points in the regression (Figures 4 and 5) were outside the predicted limit from the dataset. Points from stations 2 to 7 represented outliers for Al and Li for Co, V, Ni, Cr, Cu and occasionally for Pb and Cd. Station 20 located in the Head Lagoon was above the regional limits for V. In the Industrial Zone stations 25, 27, 29 and 34 showed anomalous levels for Ni, Pb, Zn, Cd, Co, Cu and V, and for Cd, Cu and Pb stations 36, 38 and 39 of the Navigation Channel. Adjacent to the Sewage Outfall, station 45 has anomalous levels for Zn and Co, and station 49 for Co, Cr and V. Mean Enrichment factors EFAl and EFLi were not significantly different (P<0·001) (Figure 6). Cd and Pb were above the earth’s crust, with an EF ranged from 9 to 19 times and from 5 to 4 times, respectively, which indicate

that continental weathering does not control their concentrations directly. Zn, Cu and Co were strongly correlated with Al and Li (r<0·73) with EF values between 1·5 and 4·0, which indicated that there were other natural or anthropogenic sources in addition to weathering. V and Al exhibited a positive and significant correlation with Li; these metals had an average EF close to one, and consequently they were dominantly lithogenous in origin. Cr and Ni showed low EF values (EF<1·0) which reflected a mobilization of these elements in relation to Al (Huang et al., 1992; Zhang, 1995). Mn and Fe showed an average EF close to one that confirms their natural origin. The weak correlation of Mn with Al and Li might be explained by the significant migration of Mn from reduced sediments to water column. Mangrove and lagoonal sediments The spatial distribution within the upper lagoon region was examined along two transects indicated in

268 M. Soto-Jime´ nez and F. Pa´ ez-Osuna

15

6 y 4

Cd,

Ni,

1.2

0.6

2

0

0

0.0

10

15

9

10

6

3

Cr,

y Pb,

5

y

5

5

3

0

120

24

12

80

16

8

Zn,

V 40

0

2

4

6

8

y

0

y

0

Cu,

Co,

10

8 Fe

1.8

y

8

y

20

8

0 0.5

1.0

Al

1.5 Al,

2.0

2.5

3.0

x

4

0 0.5

1.0

1.5 Al,

2.0

2.5

3.0

x

F 4. Scatter plots showing the relationships between concentrations of metals (mg kg
1) and Al (%) in surface sediments from Mazatla´ n Harbor and adjacent areas. The upper thin line represents the regression line for all data. The lower solid line represents the regression line for trimmed data while dashed lines represent 2 standard error values. Open circles represent removed points (outside the 95% prediction limits and/or >ERL); solid circle, points included in the final regression model.

Figure 1. The analysis of these transects indicated that the amount of fine material (<63
m) and the grain size parameter Mz increased towards the margins. Generally mangrove sediments contained elevated organic carbon concentrations with a more evident increase along the margin where effluents from an adjacent shrimp pond are discharged (Figures 7 and 8). Pa´ ez-Osuna et al. (1997) estimated a net load of organic material from the adjacent shrimp farm of 469 kg ha
1 for the dry season. This shrimp farm discharges effluents during the two breeding seasons in the year. Results indicated that a significant portion of this organic load was retained in the mangrove sediments located in the upper lagoon. In a single transect, it was noted that a different texture and organic carbon contents existed among the lagoonal and mangrove sediments. The tidal channels, with a high percentage of sands, showed low organic carbon and metal content (point 4 in transect A–B; and point 3 in transect A–C). In the tidal flats (points 4 to 6,

transect A–C) and small lagoon depressions, the texture and metal content were comparable to those in mangrove sediments (transect A–B). Variations in metal levels were not clear for most of the examined elements, only Ni, V and to a lesser extent Cu, showed a slight tendency to increase towards the mangrove margins (transect A–C, Figure 8). Discussion It was evident that a considerable enrichment of organic carbon in sediments occurred in most of the stations within EI and UL regions. Sources of organic matter are related to mangrove forest and input of domestic effluents in EI; and the discharge of shrimp pond effluents in UL. Mangrove forests are among the most productive ecosystems in the world; together with saltmarshes they are one of the main sources of detritus for tropical coastal lagoons (Flores-Verdugo et al., 1992). In Mexican coastal lagoons, the detritus

Distribution and normalization of heavy metal concentrations 269

15

6 y 4

Cd,

Ni,

1.2

0.6

2

0

0

0.0

10

15

9

10

6

3

Cr,

y Pb,

5

y

5

5

3

0

120

24

12

80

16

8

Zn,

V 40

0

8

16

24

32

y

0

y

0

Cu,

Co,

10

8 Fe

1.8

y

8

y

20

8

0

2

3

Li

4 Li,

5 x

6

4

0

2

3

5

4 Li,

6

x

F 5. Scatter plots showing the relationships between concentrations (mg/kg) of metals and Li in surface sediments from Mazatla´ n Harbor and adjacent areas. The upper thin line represents the regression line for all data. The lower solid line represents the regression line for trimmed data while dashed lines represent 2 standard error values. Open circles represent removed points (outside the 95% prediction limits and/or >ERL); solid circle, points included in the final regression model.

is largely composed of indigestible mangrove leaves and twigs (Flores-Verdugo et al., 1987) which represent a large carbon reservoir. Shrimp farm effluents, are associated with a significant load of organic material, chlorophyll, nitrogen, and phosphorus (Pa´ ez-Osuna et al., 1997), they are directly discharged into the mangrove sediments along the margins of the tidal channels in the UL region. The similarity of distribution patterns and the significant correlation coefficients between metal concentrations in the sediments indicated that metals might have come from the same source. The natural input of metals (including the Mn) into sediments of Urı´as estuary is derived from riverine particulate material. Seaward decreases of total concentration of most of the elements could be due to the mixing of enriched riverine particulate material with relatively clean marine sediments. It has been reported in many of the European and North American estuaries (Sinex & Wright, 1988; Zhang, 1995). The inverse tendency of Mn was probably related to differences in redox

conditions among regions. While oxygenated waters maintained a high redox level in those regions (PE, NC, IZ) where waters were more influenced by tidal currents, low redox (
31 to
204 mV) (Pa´ ezOsuna et al., 1998) were predominant and characteristics of mangrove sediments from the upper lagoon. Redox differences generating reactions involved in the diagenesis of Mn could be related to changes in solid-interstitial water phase, and could be expressed by a simple soluble Mn/solid Mn ratio. Several authors (e.g. Lynn & Bonatti, 1965; Froelich et al., 1979; Burdige & Gieskes, 1983) have described models for explaining the general conditions that control such ratio in marine and estuarine sediments. However, it is clear that when reduced conditions predominate, as commonly occurs in mangrove sediments, soluble Mn is the dominant form migrating through the porewater to the adjacent water column. When oxidized conditions predominate, Mn is present in solid forms, which is retained in the sedimentary column.

270 M. Soto-Jime´ nez and F. Pa´ ez-Osuna 18

Enrichment factor (EFLi)

15

12

9

6

3

0

Cd

Co

Cr

Cu

Fe

Mn

Ni

Pb

V

Zn

F 6. The mean enrichment factor (EFLi) for metals in surface sediments from the different regions in Mazatla´ n Harbor and adjacent areas. =Infiernillo Estuary; =Head Lagoon; =Navigation Channel; =Sewage Outfall; =Upper Lagoon; =Industrial Zone; =Port Entrance.

The resuspension-deposition pattern of Mn affects the mobilization of other elements (Shaw et al., 1990; Huerta-Diaz & Morse, 1992). In this study, concentrations of Ni and Cr from stations with anoxic sediments were lower than the background levels. This indicates that diagenetic processes associated with Mn, mobilized a significant fraction of these elements. The mobilization of these metals from sediments in reducing conditions has been found in many temperate estuaries (Zhang, 1995 and references within). The significant correlation between Co and Mn suggests that Co is scavenged by Mn oxides in sediments with oxidative conditions (Shaw et al., 1990). Our results on the spatial distribution of Fe within Mazatla´ n Harbor and a significant correlation with the reference elements (rd0·53, P<0·001) suggest that diagenetic processes have little influence on this otherwise diagenetically active element. The major processes of metal retention include cation exchange, complexing with organic molecules, precipitation as oxides, oxyhydroxides and carbonates, and precipitation as sulphides (Dunbabin & Bowmer, 1992). Sulfide precipitation is very important in mangrove sediments (Tam & Wong, 1996). Low redox potential in mangrove sediments indicates the presence of significant quantities of H2S that precipitates some metals in insoluble sulphide forms (Huerta-Dı´az & Morse, 1992). This explains the highest level obtained for Ni, V and Cu in mangrove

sediments. Harbison (1986) indicated that mangrove systems are physical traps for fine material and their transported load of metals, they also constitute a chemical trap for precipitation of metals from solution. The lack of a clear enrichment in mangrove sediments of other metals as Cd, Pb and Zn may be caused by their strong soluble complexes with reduced sulphur (Emerson et al., 1983), which will increase the migration of these elements from sediments to the water column (Huerta-Dı´az & Morse, 1992). Similar redox and texture characteristics can be found in lagoonal and mangrove sediments, resulting in a similar capacity for metal capture. Considering this, and the lack of anthropogenic source of metals in the upper lagoon and surroundings, a limited metal load produces only small (Ni, V, and Cu) or inconsistent (Cd, Co, Cr, Pb, Zn and Fe) differences in the metal content of lagoonal and mangrove sediments. Finally, the results of normalization routines showed that the stations with the highest degree of enrichment for Cd, Pb, Zn and Cu in the different regions were 6 and 7 (IE), 29 and 34 (IZ), 36 (NC) and 45 (SO). These stations had anomalous levels for the same metals that deviated by more than twice the standard error of the regional reference range established in this study. In addition, these stations presented the highest levels of outliers from the general tendency. Stations within the Infiernillo Estuary region, where sediments had elevated organic carbon,

Distribution and normalization of heavy metal concentrations 271 Mangrove

Mangrove

A

B

Shrimp pond effluents

–1

Mz 10

50

5

0

0

Cu

Ni

Pb

V 90

60

60

30

30

0

0

Fe

Cd

% Organic carbon

6

6

3

3

1

2

3

4 5 Station number

6

7

8

–1

9

Cd (mg kg )

9

0

–1

90

Metals (mg kg )

Metals (mg kg )

% Clay

100

Al Al, Fe, Corg (%)

% Silt

Grain size

%

% Sand

0

F 7. Transect A-B across mangrove-lagoonal-mangrove sediments in the Upper Lagoon, showing the distribution of organic carbon, Mz, clays, silts, sands and metal concentrations.

clay and silt content, were characterized by receiving raw sewage. It is clear that Zn and Cu concentration from stations 29 and 34 (IZ) had markedly higher values indicating anthropogenic input from industrial and/or maritime sources. Ruelas-Inzunza and Pa´ ezOsuna (1998) examined the bioavailability of Zn and Cu in barnacles; they found that specimens from the industrial zone had more Zn (2–3 times) and more Cu (5–6 times) concentration than those specimens collected from the inner area and the mouth of the lagoon. The highest value for Pb with EFPb Al and EFPb Li n1 were found at station 36, located in the Navigation Channel where tidal currents are relatively strong, but the boats traffic is intense. Also, oil wastes

from operation and accidents of the pumping and dispatching of a PEMEX plant, located adjacent to this station might contribute to these results. The particular tendency to increase of some metals (e.g. Cd, Pb, Cu, Mn and Zn) at station 45 (sewage discharge point) suggests that these metals are introduced via the sewage treatment plant. Conclusions The study showed that Al and Li could be used as reference elements to normalize concentration for most of the metals in lagoonal and mangrove sediments. Mn is an exception because redox processes

272 M. Soto-Jime´ nez and F. Pa´ ez-Osuna Mangrove

A

C

Shrimp pond effluents

–1

Mz 10

50

5

0

0

Cu

Ni

Pb

V 90

60

60

30

30

0

0

Fe

Cd

% Organic carbon

6

6

3

3

1

2

3

4 5 Station number

6

7

8

–1

9

Cd (mg kg )

9

0

–1

90

Metals (mg kg )

Metals (mg kg )

% Clay

100

Al Al, Fe, Corg (%)

% Silt

Grain size

%

% Sand

0

F 8. Transect A-C across mangrove-lagoonal sediments in the Upper Lagoon, showing the distribution of organic carbon, Mz, clays, silts, sands and metal concentrations.

play a relevant role modifying its inter-regional distribution within this coastal lagoon. The regional distribution had a strong or gradually reduced seaward gradient of fine material, organic content, and total concentrations for most of the metals, except for Mn. Similarity of spatial distribution and the strong interelement relationships suggests that common pathways and processes determine mainly the concentration and distribution for most metals. Mixing of enriched riverine material and relatively clean marine material is the main factor that controls the dispersal of the elemental concentrations. Diagenetic processes governing the different behaviour of Mn are superimposed to the mixing process. Some metals such as

Cr, Ni, Co and Fe appear to be only slightly influenced by diagenetic processes. The combination of normalization techniques confirms that most of the metals (Co, Cr, Ni, V, Al, Fe, Li and Mn) are mainly of natural origin, with the exception of some local anomalies for Cd, Zn, Cu and Pb. These anomalies are directly associated with point discharges from municipal, dock and industrial activities. The techniques described here are appropriate for assessing the metal contamination of subtropical lagoonal and mangrove sediments. The amount of fine material, organic carbon and the grain size parameter (Mz) increase towards the margins where mangrove sediments exist. However, metal variations

Distribution and normalization of heavy metal concentrations 273

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