Incidentes y Accidentes en el Juicio de Amparo.

Chalk geology and hydrogeology is summarised in Section 4.2. 106 samples were obtained from three boreholes cored into the Chalk aquifer: Layer de la Haye, Essex, sampling Upper Chalk; Banterwick Bam, Berkshire, sampling Upper and Middle Chalk; and Thriplow, Cambridgeshire, sampling Lower Chalk.

5.2.1 Upper and Middle Chalk

Upper Chalk rotary cored near Layer de la Haye, Essex, (approximate grid reference (GR) TL 971 196) sampled 108 m of chalk from 91.15 metres below ground level (m bgl), overlain by London Clay, Woolwich and Reading Beds, Thanet beds and the Bullhead Formation. The chalk was described as medium soft to hard, with putty chalk in places, 'with a decreasing number of fractures and flints with depth. The acid-insoluble carbon (AIC) content and descriptions of the samples are tabulated in Appendix D, summarised in Table 8, and illustrated in Figure 5.

A sub-sample obtained from a brown patch at 108.16 m bgl had AIC of 0.13% (excluded from Table 8), significantly higher than the adjacent white chalk material (AIC 0.024% to 0.036%). Grey chalk from 125.04 m bgl had AIC of 0.05% compared to 0.03% to 0.04% for adjacent white samples; this suggests that the grey colour observed does not relate to higher organic carbon. Samples of harder material showed no increase in AIC compared to softer samples.

Five cross-sections of core were studied to investigate whether the organic polymer drilling fluid used resulted in higher organic carbon at the core edges. No significant variation of AIC through the core was identified, indicating either that no contamination had occurred, or less probably, that a constant level of contamination permeated the core.

Chapter 5: TOC in geological material

Figure 5: AIC in Upper Chalk, Layer de la Haye

0.03 AIC (%) 0.06 0.07

— 140 -

The Banterwick Bam core (GR SU 513 775) sampled 97 m of Upper and Middle Chalk near Newbury, Berkshire; its inorganic geochemistry is detailed in Murphy et a l, 1997. The measured AIC contents are tabulated in Appendix D, illustrated in Figure 6 and summarised in Table 8. A log of the core is also included in Appendix D (Figure 68). Figure 6: AIC in Upper Chalk, Banterwick Bam

Chapter 5: TOC in geological material Table 8: Summary of AIC in Upper Chalk

Core No. of samples

analysed Mean (%) Maximum (%) Minimum (%) Layer de la Haye 39 0.039 0.065 0.015 Banterwick Bam (Upper Chalk material)

21 0.029 0.063 0.0096

Figure 5 and Figure 6 show fluctuations through the Upper Chalk cores without a clear trend, but possibly with increasing AIC with depth.

The Banterwick Bam core contained Middle Chalk below the Chalk Rock at approximately 40 m bgl. Results from AIC analyses of Middle Chalk are presented in Figure 7, summarised in Table 9 and tabulated in Appendix D.

Figure 7: AIC in Middle Chalk, Banterwick Barn

n ftrt m n i ft ft n i ft OA n a< a n a aat

Table 9: Summary of AIC in Middle Chalk

Core No. of samples

analysed Mean (%) Maximum (%) Minimum (%) Banterwick Bam

(Middle Chalk material)

46 0.027 0.068 0.013

AIC content increases with decreasing CaCOs content in the samples from Banterwick Bam (Figure 8). Decreasing CaCOs is likely to be due to increasing clay content.

Chapter 5: TOC in geological material

Figure 8: Negative correlation of AIC with CaCOj, Banterwick Barn

0.07 0.02 0.01 82 84 86 88 90 92 94 96 98 100 ♦ U p p e r C h a lk o M id d le C h a lk 5.2.2 Lower Chalk

Lower Chalk material was obtained from the Thriplow core from Cambridgeshire (GR TL 446 446). AIC content and descriptions of these Lower Chalk samples are tabulated in Appendix D, illustrated in Figure 9 and summarised in Table 10.

Figure 9: AIC in Lower Chalk, Thriplow

n n n n mn m n aa a a p a^a a<a a^j

These data indicate an increase in AIC content with depth, which may be associated with increasing clay content with depth in the Lower Chalk.

Chapter 5: TOC in geological material

Table 10: Summary of AIC in Lower Chalk

Core No. of samples

analysed Mean (%) Maximum (%) Minimum (%) Thriplow 52 0.045 0.12 0.007

5.2.3 Previously published work

Whitelaw and Edwards (1980) and Pacey (1989) analysed various organic components from Upper and Middle Chalk samples, although neither report TOC content. Whitelaw and Edwards (1980) found carbohydrate^^ content in the Middle Chalk of 0.0065% to 0.03%, with an exponential decrease with depth. Upper Chalk samples had carbohydrate concentrations between 0.0002% and 0.02%. It was proposed that the carbohydrates were products of bacterial metabolism and suggested that variations between sites may be related to land use. Pacey (1989) found humic matter^^ to be between 0.01% in white chalks and 0.1% in clayey and phosphatic chalks. Fulvic acid content was estimated to be up to 20% of the humic matter. Less than 30% of the humic matter was amenable to direct extraction, the bulk (and all the bitumen'^) being obtained after dissolution of the matrix. This was thought to indicate a diffuse distribution of organic matter in the chalks, with some intra-crystalline within the calcite, and some associated with the clays. Bitumen levels were 0.01% to 0.15%, usually similar to the humic acid content in the samples. Only small amounts of kerogen*"^ were foimd, as most of the conventional kerogen fraction was found to be alkali soluble after matrix dissolution.

Bein and Sandler (1983) determined organic carbon content with a LECO® analyser on the residue of hydrochloric acid dissolution from a series of Eocene chalks and cherts from Israel. The organic matter content of the chalks was found to be higher than that of chert, but analysis of extracted kerogen and humic acid indicated that the composition of

Carbohydrate has many hydroxyl functional groups.

Humic matter is the fraction of sedimentary OM that is soluble in basic solutions, and comprises both fulvic acids that are also soluble in acidic solutions and humic acids that are not.

Bitumen is organic material insoluble in basic solution and soluble in nonpolar organic solvent (Pacey, 1989).

Kerogen is organic matter in sediments that consists o f large molecules and is insoluble in non-polar organic solvents.

Chapter 5: TOC in geological material

the organic matter was similar. Total organic carbon ranged between 0.16% and 0.63% in the cherts, and 0.38% and 1.77% in the chalks.

5.2.4 Summary of TOC in Chalk

Upper and Middle Chalk have AIC content less than 0.1% (usually between 0.01% and 0.05%), fluctuating through the depth of the cores studied, showing no consistent trends. Locally higher AIC content appeared to correlate with small brown areas and with decreased CaCO] content. Lower Chalk has higher organic carbon content than Upper and Middle Chalk (AIC up to 0.12%), associated with higher clay content and darker appearance. Chalk AIC content is relatively low; organic carbon in the Chalk, especially in Upper and Middle Chalk, is unlikely to provide a significant sorbent.