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Lateral macropore dominated flow on a clay settling area in the phosphate mining district, peninsular Florida
h [electronic resource] /
by Natalie Pechenik.
[Tampa, Fla] :
b University of South Florida,
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Document formatted into pages; contains 33 pages.
Thesis (M.S.)--University of South Florida, 2009.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
ABSTRACT: The objective of this study was to use an applied tracer to study lateral ground water flow paths in the top ~0.5 m of clay settling areas (CSA) in order to gain better understanding of hydrologic connectivity of CSAs to the surrounding hydrologic systems. The study site was located on the non-operational Mosaic Fort Mead Mine property in Fort Meade, Polk County, Florida. This lateral tracer test study is a follow up from a vertical tracer test study performed at the same site location in 2007. The CSA is generally composed of a well developed, clay rich, subangular-blocky surface layer ~0-1.0m, which exhibits abundant desiccation cracks plus other macropores underlain by a massive, saturated, clay-rich sublayer from ~1.0-2.5 m. A bromide tracer was applied into an injected trench. All 60L of the applied tracer flowed out of the down gradient face of the trench quickly, over an eleven minute period.The Bromide tracer was rapidly transported laterally and was detected as far as 16 m from the starting point just 24 hours after application, as well as in the inundated north pond adjacent to the study area. Bromide concentration distribution was not uniform over the study area during any time period, with an initial disorganized bromide pulse followed by secondary pulse concentrated on the north side of the sampling area. This spatial-temporal distribution of bromide indicates preferential flow through desiccation cracks or other macropores. Bromide concentrations in the north pond increased over time while pond stage fluctuated due to this shallow lateral macropore dominated flow in and out. Although it is most likely true that flow paths from the CSA to the adjacent hydrologic landscape during the wet season is dominated by rapid shallow lateral flow through macropores, specific flow paths, macropore length, diameter and distribution and fluxes still remain unquantified. Therefore, how the hydrology of CSAs affects the adjacent hydrologic landscape still remain unquantified.
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Advisor: Mark Cable Rains, Ph.D.
t USF Electronic Theses and Dissertations.
Lateral Macropore Dominated Flow On A Clay Settling Area In The Phosphate Mini ng District, Peninsular Florida by Natalie Pechenik A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science Department of Geology College of Arts and Sciences University of South Florida Major Professor: Mark Cable Rains, PhD Mark Stewart, PhD Mark Ross, PhD Date of Approval: July 2, 2009 Keywords: groundwater hydrology geochemical tracer, desiccation crack, perched aquifer preferential transport Copyright 2009 Natalie Pechenik
i TABLE OF CONTENTS LIST OF FIGURES ii ABSTRACT i ii INTRODUCTION 1 SITE DESCRIPTION 4 Location and Setting 4 Climate 4 Hydrogeology 8 Vegetation 9 METHODS 1 0 Physical Hydrolog ical Measurements and Analysis 1 0 Tracer A ppl ication and Sample Collection 1 1 Laboratory Analysis 1 2 RESULTS 1 3 Applied Tracer 1 3 North Pond Stages and Groundwater Hydraulic Heads 1 4 DISCUSSION 2 3 CONCLUSIONS 2 5 REFRENCES 2 6 APPENDI CI ES 3 2 Appendix i. List of bromide concentrations for core samples in mg/L 3 3
ii LIST OF FIGURES Figure 1. Plan view of the study site 6 Figure 2 Fence diagram through the CSA and surrounding hydrological landscape 7 Figure 3 Rainfall in cm/d at the CSA immediately before, during, and after the tracer application 1 5 Figure 4a Postplot of bromide concentrations over simple fan sampling area for day 1. 1 6 Figure 4b P ostplot of bromide concentrations over simple fan sampling area for week 1 1 7 Figure 4c Postplot of bromide concentrations over simple fan sampling area for month 1 1 8 Figure 4d Postplot of bromide concentrations over simple fan sampling area for month 3 1 9 Figure 5 Bromide concentra tions over time in the north pond 2 0 Figure 6 Precipitation in cm/d and T1 hydraulic head levels over time 2 1 Figure 7 Precipitation in cm/d and north pond stage above land surface over time 2 2
iii Lateral Macropore Dominated Flow on a Clay Settling Areas in the Phosphate Mining D istrict, Peninsular Florida, Natalie E. Pechenik ABSTRACT The objective of this study was to use an applied tracer to study lateral ground water flow paths in the top ~0.5 m of clay settling areas (CSA) in order to gain better understanding of hydrologic connectivity of CSAs to the surrounding hydrologic systems. The study site was located on the non operational Mosaic Fort Mead Mine property in Fort Meade, Polk County, Florida. This lateral tracer test study is a follow up from a vertical tracer test study performed at the same site location in 2007 The CSA is generally composed of a well developed clay rich subangular blocky surface layer ~ 0 1.0 m which exhibits abundant desiccation cracks plus other macropores underlain by a massive, saturated, clay rich sublayer from ~1.0 2.5 m A bromide tracer was applied into an injected trench All 60L of the applied tracer flowed out of the down gradient face of the trench quickly over an eleven minute period. The Bromide tracer was rapidly transported laterally and was detected as far as 16 m from the starting point just 24 hours after application, as well as in the inundated north pond ad jacent to the study area Bromide concentration distribution was not uniform over the study area during any time period with an initial disorganized bromide pulse followed by secondary pulse concentrated on the north side of the sampling area. This spatial temporal distribution of bromide indicates preferential flow through desiccation cracks or other macropores. B romide c oncentrations in the north pond increased over time while pond stage fluctuated due to this shallow lateral macropore dominated flo w in and out Although it is most likely true that flow paths from the CSA to the adjacent
iv hydrologic landscape during the we t season is dominated by rapid shallow lateral flow through macropores, specific flow paths macropore length, diameter and distribution and fluxes still remain unquantified Therefore, how the hydrology of CSAs affects the adjacent hydrologic landscape still remain unquantifie d.
1 INTRODUCTION Geologic deposits composed of clay rich materials typically have low permeabilities (Fetter, 2001) and are typically trea ted as homogeneous and isotropic deposits (Dekker and Bouma, 1984) Low permeability, clay rich deposits can perch or confine aquifers (Fetter, 2001; Rains et al 2006) and therefore are often used to retard or restrict flow in or out of solid waste disposal sites and well annuli (Johnson et al., 1989; Sanders, 1998). However, t h e assumption that clay rich deposits are homogeneous and isotropic is not necessarily correct in clay rich deposits with desiccation cracks and other macropores ( T homas and Phillips, 1979; Bouma et al., 1981; Dekker and Ritsema, 1996 ). Clay rich deposits have the capability to swell when wetted and shrink when dried due to interlayer water adsorption within the clay (Tuller and Or 2003). When subjected to repeat n ear surface wetting and drying events clay rich deposits can form well defined desiccation crack s The p hysical attributes of the void spaces are variable due to differences in mineralogy, degree of compactio n, grazing or nature of wet and dry cycles (Reid and Parkinson, 1984; Vogel et al., 2005) D esiccation cr acks caused by wet/dry cycles or other bioturbation macropores such as root channels or burrows within a clay rich deposit can generate preferential flow pat hs ( Thomas and Phillips, 1979; Boum a et al., 1981; Dekker and Ritsema, 1996 ) Although desiccation cracks and other macropores comprise a small proportion of the total porosity of a clay rich deposit they nevertheless can cause substantial enhancements to flow paths enabl ing water to bypass the low permeability matrix and allow for rapid flow across the landscape (Bouma and Dekker, 1978) Therefore desiccation cracks and other macropores can easily dominate and be responsible for
2 much of the flow and transport through the sha llow subsurface of a clay rich deposit (Heppell et al., 2000) Clay settling areas (CSAs) are clay rich deposits which are a byproduct of phosphate mining in peninsular Florida The mineral resource typically lies beneath ~4 8 m of overburden and is ~3 4 m in thickness and typically is composed of one third various sized phosphatic sediment, one third sand sized sediment and one third clay sized sediment (Florida Institute of Phosphate Research, unpublished data) One byproduct of the phosphate beneficiation processes is a clay rich slurry which is composed o f ~3% solids, typically has a d 50 of < 0.2 microns (Hawkins, 1973) This clay slurry is pumped to CSAs, which are reservoir s contained by earthen berms ~6 15 m above grade and ~120 320 hecta res in size (Lewelling and Wylie, 1993; Zhang and Albarelli, 1995) Once pumped into the CSA, the clay slurry quickly consolidates and excess water is drained off resulting in ~18 22% solids within a few months and a surface crust that is ~50 60% solids after 5 years although the deeper subsurface will remain ~25% solids for an unknown amount of time (Zhang and Albarelli, 1995; Erwin et al., 1997) Phosphate mining operations are still active in peninsular Florida. A s of 1998 there w ere ~120,000 hecta res of mined land, ~50,000 hectares of which was covered with CSAs (Stricker, 2000) Over the life span of all phosphate mining operations the total mined land could be ~175,000 hectares of mined land, ~70,000 hectares of which could be covered with CSAs There no typical CSA s, in that CSAs v ary in size, thickness, height above grade, proximity to other CSAs, and/or age (Murphy et al., 200 8 ) Despite their abundance, little research ha s been done on the hydrology of CSAs and their effects on surrounding hydrologic systems and water resources. CSAs were long conceptualized as being precipitation sinks, with large evapotranspiration outflows, minimal surface water outflows and negligible groundwater outflows. This capture zone concept implies that the effective size of a drainage basin is reduced due to the presence of a CSA yet little
3 research has been done to quantify this theory. Eve n basic hydrologic processes of CSAs are still poorly understood (Lewelling and Wylie, 1993; Zhang and Albarelli, 1995 ; Murphy et al. 2008 ) This study is part of a larger four year study of CSAs and supplemental to a previous hydrologic investigation of one particular CSA. The preceding study performed by Murphey et al. (2008 ) indicated that the simple assumption that CSAs are precipitation sinks, with large evapotranspiration outflows, minimal surface water out flows and negligible groundwater outflows may be incorrect Instead, Murphy et al. (2008) suggest ed that CSAs possess perched surface water and groundwater flow systems and deep groundwater flow systems, with both flow systems discharging into the surrounding hydrologic landscape However unanswered questions remained, including specific details of the mechanisms and flow paths by which water flows through the p erched surface water and groundwater flow systems T he objective of this study was to answer some of those questions, by using an applied tracer and shallow monitoring wells to better understand the mechanisms and flow paths by which water flows through th e perched surface water and groundwater flow systems.
4 SITE DESCRIPTION L OCATION AND SETTING The study site is located near Fort Meade, Polk County, Florida (Figure 1). The CSA is ~20 years old and is ~ 6 m above grade and ~75 hectares in size The surface crust of the CSA is well developed and can support vehicles during both the wet and dry seasons. Though the surrounding area was mined, the CSA in question is located on un mined land so the bottom boundary conditions are reasonably well understood The surface of the CSA slopes slightly from north to south with the surface varying from nearly level to undulating (Figure 2) The southwest face of the earthen berm slump s slightly, generating a slight slope towards the southwest corner of t he CSA and the adjacent field. CLIMATE The climate at the study site is sub tropical with relatively hot, humid summers and warm dry winters. Summer rainfall is frequent and can be of high intensity and is typically due to locali zed convective thunders torms while winter rainfall is less frequent and is typically due to continental cold fronts (Lewelling and Wylie, 1993) Mean ( standard dev iation) annual precipitation is 135.3 ( 24.5) cm, with ~65% falling during the months of May September (Southeastern Regional Climate Center data for Bartow, Florida for calendar years 1931 2006). Annual precipitation in the area was 103 cm for water year 2008 so rainfall was lower than
5 normal in during the course of this study (Southeastern Regional Clima te Center data for Bartow, Florida for water year 2008 ).
6 Figure 1. Plan view of the study site
7 Figure 2. Fence diagram through the CSA and surr ounding hydrological landscape. Points in the inset and on the cross section are in the same locations. Vertical exaggeration is ~20x.
8 HYDROGEOLOGY There are three principal hydrostratigraphic units that compose the central Florida phosphate mining district: the surficial aquifer, the intermediate aquifer system and the deep Floridan aquifer system (Erwin et al., 1997) The phosphate rich sediment is mined from the upper part of the intermediate aquifer in the Bone Valley Member which is located within the Miocene aged Hawthorn Fo rmation. The berms of the CSA are composed of sand rich overburden from the local surficial aquifer. The clay slurry deposited into the CSA is composed of clay from the Bone Valley Member mixed with local ground water and other processing waters (Reigner and Winkler, 2001) Currently, t he CSA is generally com posed of a well developed, clay rich, subangular blocky surface layer ~ 0 0.5 m which exhibits abundant desiccation cracks plus other macropores underlain by a massive, saturated, clay rich sublayer f rom ~ 0.5 2.5 m (Murphy et al. 200 8 ). This observation is based upon 38 boreholes, two shovel test pits, and two front loader test pits at various locations on the CSA. Based upon observations at six deep boreholes t he massive, saturated, clay rich sublayer is ~5 6 m in thickness and underlain by un mined sands of the surficial aquifer. Although there are no surface water inflows, there are surface water outflows via engineered culverts through the east berm. There may also be groundwater outflows through the berms (Murphy et al. 2008). The CSA surface has at least seven closed basin dep ressions of various size and shape that seasonally pond with water. The closed basin depressions on the CSA have no surface water inflows or outflows, so surface water levels are controlled by precipitation evapotranspiration, and groundwater throughflow (Murphy et al, 200 8 ).
9 VEGETATION Invasive Cogan Grass, ( Imperata cylindrical (L.) Raeuschel ) composes upwards of ~65% of the vegetation on the CSA. The remaining ~35% of the CSAs surface is composed of mixed hardwoods. The uplands exhibit the invasive Earleaf Acacia ( Acacia auriculiformis A. Cunn. Ex. Benth) while the seasonally inundated closed depressions consist of native Coastal Plain Willows ( Salix floridana Chapm.)
10 METHODS Both physical and chemical hydrological methods were used for this study All data w ere collected during the 2008 calendar year. PHYSICAL HYDROLOGIC MEASUREMENTS AND ANALYSIS Precipitation was measured continuously with an ET106 Weather Station (Campbell Scientific, Inc., Logan, Utah) On site weather station data w ere not available from January 1 March 1 2 at 10:00 AM due damage from a wild fire that occurred the previous year Therefore, precipitation data were only available beginning March 12 at 10:00 AM Hydraulic head in the shallow subsurface of the upland (hereafter referred to as T1) and in a down gradient closed basin depression (hereafter referred to as the north p ond or W1 ) w ere measured at 10 minute intervals with pressure transducer s and datalogger s ( Solinst Can ada, Ltd., Georgetown, Ontario, Canada ) T1 and W 1 both had 5 cm inside diameter PVC stand pipe s T1 was installed in a 100 c m wide, 30 cm long, and 50 cm deep trench backfilled with sand and capped with a cement surface seal to ensure that measured hydrau lic heads were representative of mean macropore hydraulic heads. T1 was screened from 25 50 cm below the ground surface. The blank tip of the screen was below 50 cm, so any measured water levels >50 cm below the ground surface was due to residual water. W1 was installed in a 30 cm diameter borehole backfilled with sand but not capped with a cement surface seal.
11 TRACER APPLICATION AND SAMPLE COLLECTION Bromide was used as an applied tracer to study shallow vertical and lateral ground water flow paths. Bromide was chosen because background concentrations were naturally low (McCutcheon et al., 1993 ; Murphy et al. 2008 ). A 100 cm wide, 25 cm l ong and 50 cm deep trench was dug perpendicular to the apparent flow path o n the northern part of the CSA All fa ces of the trench excluding the down gradient face were lined with plastic to direct all flow down gradient. On July 21 at ~1:00 PM 60 L of 50 g/L LiBr solution was introduced into the trench. A ll 60L of the solution infiltrat ed in ~11 minutes. T he trench was then backfilled with parent material. Porewaters were collected from core samples taken with an open barrel hand auger at 0, 2, 4, 8 and 16 m away in a 45 fan shape from the trench. Cores were augured no deeper than 0.5 m and were placed into wide mouthed 300 mL HDPE sample bottles One sample was collected at 0 m, three samples were collected at 1 m, five samples were collected at 2 m, seven samples were collected at 4 m, nine samples were collected at 8 m, and 11 samples were col lected at 16 m An additional surface water sample was collected from the north pond just beyond the 16 m sample area Samples were collected five time s in total: the initial day of the injection (July 21 ) one day after the injection (July 22 ) one week after the injection (July 29 ) one month after the injection (August 22) and three months after the injection (October 27) After each hole was augured, it was backfilled with native material to minimize down borehole contamination. A hydraulic sedimen t sq ueezer was used to extract porewaters from the core samples (Manheim et al., 1994) Core samples were subjected to 200 250 psi. Extracted porewaters w ere filtered through two 0.45 m in line filters and collected in 10 mL syringes After at least 1.5 mL of pore water was collected in the syringe, porewater sample s w ere transferred to 30 mL HDPE sample bottle s Each sample took approximately 30 60 minutes to extract and was composed of a
12 minimum of 1.5 ml and a mean of 1.8 mL of water Surface water fro m the north pond was filtered through one 0.45 m in line filters and placed directly into 30 mL HDPE sample bottle s LABROTORY ANALYSIS Bromide analyses were conducted at the University of South Florida Center for Water Analysis. Bromide concentrations were determined by ion chromatography following the EPA 300 method (Clesceri et al., 1998). All samples were kept at the method required 4 C prior to analysis. Analytical preci sion of the laboratory analyses were better than 1%.
13 RESUL TS APPLIED TRACER No rainfall occurred between the injection and the day one sample collection, but rainfall began soon thereafter and persisted episodically throughout the duration of tracer test (Figure 3 ). Despite the lack of rainfall between the injection and the day one sample collection, t he b romide was nevertheless transported rapidly across the CSA immediately following the injection (Figure 4) At day one the bromide was detected 16 m from the injection, with concentrations largely focused t oward the left side of the sample fan. At week one and month one bromide was still detected at 16 m from the injection, with concentrations more focused toward the right side of the sample fan. A t month three bromide continued to be detected, though conc entrations were lower but still spread non uniformly through out the sample fan Meanwhile, b romide concentrations increased steadily over time in the north pond, just down gradient of the 16 m sample area ( F igure 5). At no point during the three month sampling period was distribution of bromide concentration over the entire sample area uniform. Mean ( standard deviation) bromide concentrations over the entire sample area were 1.4 2 ( 1.5 9 ) mg/L on day one 10.0 1 ( 44.81 ) mg/L on week one 3.0 2 ( 4.36 ) mg/L on month one, and 2.93 ( 6.4 6 ) mg/L on month three Minimum and maximum bromide concentrations over the entire sample area were found on day one and week one respectively.
14 NORTH POND STAGES AND GROUNDWATER HYDRALIC HEADS From June 1 June 25 there was 17 cm of total rainfall. However, all of this rainfall was either lost to evapotranspiration or went into shallow groundwater storage as there was no ground water in T1 or W1 in response to any of these rainfall events From June 26 28 5 59 cm of rain fall occurred with ground water levels in W1 and T1 finally rising (Figure s 6 and 7) Thereafter, both T1 and W1 responded rapidly to rainfall, with rapid hydraulic head rises and falls, indicating rapid infiltration and lateral flow through the shallow subsurface. From June 1 October 27, minimum and maximum stages at T1 were 49.1 cm below lands surface (October 10) and 3.64 cm above lands surface (August 19), respectively and at W1 were 21.03 cm (July 7) and 78.63 cm (August 31) above the gro und surface, respectively. Rapid macropore flow was evident in T1, where hydraulic heads less than or equal to 50 cm below lands surface rose and fell rapidly in response to rainfall. After two consecutive precipitation events on June 26 and 27, a total of 5.58 cm of precipitation fell and the hydraulic head increased by 41.75 cm to 8.25cm below lands surface by June 28. The next day, June 29, no precipitation was recorded, and the hydraulic head dropped by 15.21cm to 23.49 cm below lands surface. The day after that, June 30, a subsequent drop of 9.75 cm was recorded with a total head of 33.24 cm below lands surface. In total the pressure transducer at T1 recorded a head drop of ~24.99 cm below lands surface over a 48 hour precipitation free period during which storage was already satisfied. This magnitude of head drop is greater than would be expected from ET, which according to Murphy et al (2008) is ~0.5 ( 0.1) cm/d
15 Figure 3. Rainfall in cm/d at the CSA immediately before, during, and after the tra cer application. Arrows indicate core sampling dates on Day 0, Day 1, Week 1, Month 1, and Month 3, respectively.
16 Figure 4a. Postplot of bromide concentrations over simple fan sampling area for day 1. The size of the circles is proportional to bromide concentration.
17 Figure 4b. Postplot of bromide concentrations over simple fan sampling area for week 1. The size of the circles is proportional to bromide concentration.
18 Figure 4c. Postplot of bromide concentrations over simple fan sampling area for month 1. The size of the circles is proportional to bromide concentration.
19 Figure 4d. Postplot of bromide concentr ations over simple fan sampling area for month 3. The size of the circles is proportional to bromide concentration.
20 Figure 5. Bromide concentration s over time in the North Pond.
21 Figure 6. Precipitation in cm/d and T1 hydraulic head levels over time
22 Figure 7. Precipitation in cm/d and north p ond stage above land surface over time
23 DISCUSSION The Murphy et al. (200 8 ) ha d previously suggested that shallow subsurface ground water flow and transport and the seasonal inundation of closed basin depressions on the CSA were controlled by rapid lateral flow through desiccation cracks and other macropores This assumption was based upon a vertical tracer test performed at the same CSA. During the vertical tracer test bromide concentrations decreas ed over time without penetrating to depths greater than ~0.5 m at any time and with only a minor amount of bromide uptake attributed to plant uptake (Whitemer et al., 2000; Murphy et al., 2008) During the initial phases of the lateral tracer test it was evident that some flow path other than flow through the clay matrix alone was responsible for flow through the upper ~0.5 m of the CSA. All 60L of the 60g/L Li Br solution infiltrated in ~11 minutes and was detected up to 16 m away just one day later This could not have occur red throu gh the clay matrix alone, which has an estimated hydraulic conductivity of 10 5 10 7 m/d (Morris and Johnson, 1967; Davis, 1969) Flow through preferential flow paths, such as desiccation cracks and other macropores, must have occurred. The sampl e fan slight sloped from northeast to southwest, and although bromide concentrations indicate d flow and transport over the entire sample f an concentrations were not uniform or typical of flow and transport though a homogeneous and isotropic medium. One day afte r the injection there was a disorganized initial pulse with bromide transport in multiple down gradient directions, but with more bromide focused toward the left side of the sample fan One day and one month after the injection, there was a second pulse, with more bromide focused toward the right side of the sample fan. Three months after the injection, concentrations were
24 lower and still non uniformly distributed throughout the sample fan The s e inconsistencies may have been due to changes in soil moisture content, resulting in swelling or clays and changing in the distribution and dimensions of desiccation cracks and other macropores This rapid macropore flow moves groundwater rapidly into the closed basin depressions on the CSA. Although low, bromide concentrations were nevertheless detected in the north pond just one day after the injection and continued to increase throughout the course of the three month test This confirms the hypothesis of Murphy et al, (200 8 ) who had suggested that flow throug h macropores controlled stages in the north pond based upon the results of a water balance which showed that pond stages rose and fell more rapidly than could be due to precipitation and evapotranspiration alone These results indicate that p referential flow through desiccation cracks and other macropores dominates flow and transport in the perched groundwater flow system in the upper ~0.5m of the CSA. In the early wet season, rainfall infiltrates and is either lost to evapotranspiration or is taken into soil moisture storage. Once soil moisture storage is filled, subsequent rainfall infiltrates rapidly perches on top of the massive clay unit at ~0.5 m in depth and flows laterally through desiccation cracks and other macropores. These flows can then intersect and flow through the close d basin depressions, which are depressional features inset into the shallow, perched groundwater flow system
25 CONCLUSION S The results of this study indicate that older CSAs can support shallow, perched groundwater flow systems in which shallow groun dwater flow is dominated by flow through desiccation cracks and other macropores Interconnected desiccation cracks and other macropores that form over time in the top ~0.5 m of the CSA due to shrinking swelling and subsequent cracking of the clays, root channels and/ or bioturbation are th e specific fast flow paths responsible for th e observed efficient flow and transport As suggested by Murphy et al. (2008), these rapid flows largely control stages in the seasonally inundated closed basin depressions on the CSA s Exact desiccation crack and other macropore length, diameter and distribution and the effect of the desiccation cracks and other macropores on the effective hydraulic conductivity of the upper 0.5 m of the CSA, remain unqu antified.
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32 LIST OF APPENDICIES
33 Appendix i. List of bromide concentrations for core samples in mg/L.