Historical fluxes of trace organic compounds

Profiles of a- and ~-HCHs in lakes 6 and 7 are illustrated in figure 5.23. Concentrations of the sum of HCHs in lake 6 range from 0.04 to 0.8 ng/g and from 0.42 to 1.38 ng/g in lake 7. The sum of HCHs (a+

~) in lake 7 decreases from the top of the core to the bottom, while in lake 6 the highest concentration is between 5 and 7 g/cm² of cumulative dry weight and concentrations decrease below (fig. 5.23). While a-HCH is the dominant a-HCH in lake 6, ~-HCH is slightly more abundant in lake 7 than a-HCH (fig. 5.23).

Lindane (y-HCH) was detected in only two samples in lake 6 with concentrations lower than 0.27 ng/g.

In lakes 3 and 5 in the outer delta, concentrations of a-HCH are less than 0.28 ng/g (appendix 4.5c).

In bath cores, ~-HCH has not been detected except in lake 5 at the bottom of the core (0.67 ng/g). The differences of concentrations and a- and ~-HCH distribution in the four lake cores cannat be explained at this time. It may result from problems during the analytical procedure or from natural processes in delta lakes. Lindane (y-HCH) was detected in only one sample (0.15 ng/g) in lake 5.

In overbank sediments, a-HCH was detected in all levees, except in levees 3 and 7. Concentrations are lower than 0.23 ng/g (appendix 4.1). Concentrations in surface sediments are usually similar to the ones measured in samples between 6 and 8 cm of depth.

Among the various compounds considered as cyclodienes (table 2.1 0), en drin and dieldrin were occasionally detected in lake 7 at very low concentrations ( < 0.37 ng/g.), white in lake 6 no cyclodiene was detected. a-endosulfan was also detected in surface sediments in lake 7 (1.8 ng/g). In the outer delta, in lakes 3 and 5, a-chlordane, dieldrin and heptachlor were occasionally detected but their concentrations are always smaller than 0.32 ng/g. No cyclodiene was detected in overbank sediments.

5.3.3 Historical flu xes of trace organic compounds

As described in cbapter 4.3.2, the general flux equation (Fz=o) has been simplified by neglecting the

fluxes due to molecular diffusion and to advection of pore water as the concentration of the chemical species in water is supposed to be small compared to the concentration on solid sediment particles. Then, the flux of the species to the sediment is mainly due to the deposition of solid particles (LERMAN, 1979):

Fz=0 = Rs Cg Ps (1-<j>) (g/cm²*y) where Cg Ps (1-<j>) =Cs <j> and

Cg is the concentrat~on in solids in units of mass per unit mass of solids [glg] and Cs is the concentration in solids expressed in g/cm³ of pore water. Ps is the solid densily [glcm³] and <1> is the sediment porosity. Z is the vertical distance coordinate [cm] and is positive downward and Rs the sedimentation rate. Rs is positive when the sediment and water flow downward relative to the sediment water interface (z = 0).

Fluxes due to molecular diffusion and to advection of pore water cannat be calculated as pore water in lake and overbank sediments has not been analyzed. However, the dissolved flux has been neglected as the organic compounds analyzed in this study are hydrophobie and preferentially adsorbed to particles (CAREY et al., 1990).

Fluxes of hydrocarbons, PCBs and chlorobenzenes into connected lakes and on distributary channel levees are calculated by multiplying the sediment accumulation rates [g/cm²*y] by the concentrations of these compounds.

Organic compounds fluxes have been calculated since 1954 in lakes 6 and 7 and in levee 8 where the sedimentary record is assumed to be complete (see chapters 3.2.4 and 3.3.4). The sedimentatlon rates used here have been determined in chapters 3.2.3 and 3.3.3. When samples are older than 1954, the sedimentation rates determined between 1954 and 1993 have been extrapolated to the bottom of the core.

Profiles of PCB and organochlorine pesticide fluxes in overbank sediments are not available as only two samples per core were analyzed.

Hydrocarbon fluxes

Time-dependent fluxes of alkanes and PAHs in lakes 6 and 7 are presented in figure 5.24. In lake 7, two periods of high PAH fluxes occurred, the first one in the earl y 1980s ( 1.5 ng/cm²*y) and the second one in the early 1990s (1.4 ng/cm²*y) (fig. 5.24). In lake 6, the PAH flux has been quite constant since the 1950s but a small increase can be observed during the early 1980s and 1990s (0.51 ng/cm²*y). On levee 8, the PAH flux increased from the early forties until 1965 (1 ng/cm²*y) and decrease afterwards until 1993 (fig. 5.24).

The profile of the sum of n-alkanes indicates that severa! periods of high flux occurred since the 1950s (fig. 5.24). In lake 6, the periods between 1955 and 1963, between 1966 and 1970 and between 1975 and 1982 are characterized by high flux of n-alkanes (between 9 and 13 nglcm²*y). Like in lake 6, the n-alkane flux in lake 7 is high during the early 1960s and 1970s (between 32 and 36 nglcm²*y). The flux increased again during between 1988 and 1990. On levee 8, the trend of n-alkane flux is similar to that of PAH flux (fig. 5.24). The flux of n-alkanes increased from the early forties until 1974 (24 ng/cm²*y) and decreased afterwards until 1993.

PCB fluxes

The flux of the sum of PCBs in lake 7 increased from the early 1960s until 1993. On average, the PCB flux is higher between 1985 and 1993 (10 ng/cm²*y) than before 1985 (5 nglcm²*y). In lake 6, the trend of PCB flux observed in lake 7 is Jess obvious. The higher flux observed since 1955 occurred between

1989 and 1992 (1.9 ng/cm²*y) (fig. 5.24).

-141-Chapter 5: Recent Historical Deposition of Trace Organic Compounds Organochlorine pesticide fluxes

Among the organochlorine pesticides measured in lake sediments, only profiles of chlorobenzene fluxes can be determined as the other pesticides have been detected only occasionally. In lakes 6 and 7, the flux of chlorobenzenes increased from 1955 until 1985 or 1986 (from 0.8 to 5.2 ng/cm²*y in lake 6 and from 0.9 to 4.4 in lake 7), decreased between 1986 or 1987 and 1992 and increased dramatically between 1992 and 1993. Values of chlorobenzene fluxes are similar in lakes 6 and 7, while fluxes of hydrocarbons and PCBs are higher in lake 7 than in lake 6.

Levee 8

PAH flux [Jlg/cm²*y] nC13-C30 flux [Jlg/cm²*y]

0.0 0.5 1.0 1.5 2.0 0 5 10 15 20 25

1990 1990

1985 1985

p

1980 ~

1980

f:==:l

1975 1975

1970 1970

~ 1965 1965

Ill

Q) >- 1960 1960

1955 1955

1950 1950

1945 1945

1940 1940

1935 1935

Figure 5.24: Time-dependen.t fluxes of the sum of PAHs, n-alkanes, PCBs and chlorobenzenes in lakes 6 and 7 and in levee 8 (middle delta). Comparison with production and sales of PCBs and chlorobenzenes in the U.S. (EISENREICH et al., 1989).