Geology Reference
In-Depth Information
this work suggest the existence of connections to the
surface or other caves which allows large-scale
natural
dyes with the aim of tracing connections between
infiltrating rainwater and points where seepage
enters the cave. Following methodology described
by Smart (1976) and Bottrell & Atkinson (1992),
two aliquots of the fluorescent dye Photine CU
were simultaneously released into excavated fis-
sures in bedrock at nearby points above the
Gib04a site in the cave. A second dye, Diphenyl
Brilliant Flavine DY96, was washed into scree at a
third point at a higher altitude, located where the
dip of the Gibraltar limestone projected upwards
from Gibo4a in the cave intersects the surface near
the summit ridge (Fig. 3). The injections were
made in March 2007 during a period of high rainfall,
and the dyes were washed into the ground by runoff
from sheets of polythene placed for this purpose.
Within the cave, a network of cotton detectors
were deployed at 27 stations. These provided
regular monitoring for both tracers for 85 days
after the date of injection, as well as a period of
background monitoring for 59 days beforehand.
advective
transport
of
air
through
the
entire system.
Cave environmental monitoring
All meteorological data were recorded at the RAF
Meteorological Office located 3 km away from the
cave (Fig. 1). The cave monitoring, sampling and
analysis program obtained data via continuous
logging (l) and spot measurements or sample(s)
taken during regular monthly visits from June
2004 - April 2008, for the following parameters:
1. cave air: pressure (l), temperature (l) and
relative humidity (l,s), mixing ratios of CO
2
(l,s) and CH
4
(s), analysis of d
13
C
CO2
and
d
13
C
CH4
(s)
2. soil: temperature (l) and soil air mixing ratios
of CO
2
and CH
4
(s), analysis of d
13
C
CO2
and
d
13
C
CH4
(s)
3.
drip water: discharge (l); pH, total alkalinity,
electrical
Drip water discharge rates, sampling and
analysis
cations, d
18
O
conductivity,
and
dD (s)
The locations of environmental monitoring sites are
shown on the cave plan in Figure 2. Seasonal vari-
ations in temperature and relative humidity in
ambient air at the cave entrance, within the main
chambers of the cave system and within the soil
zone above the cave (temperature only) were
measured by continuous logging supplemented by
spot measurements made using hand held instru-
ments during monthly visits. Temperature and
humidity were measured by continuous logging
using single- (temperature) and dual-channel (temp-
erature and humidity) loggers from Gemini Data
loggers deployed outside above the cave entrance
tunnel, and in the Hospital, Dark Rift and Gib04a
sites within the cave (Fig. 2). Soil temperature at
50 cm depth was logged at the soil CO
2
sampling
site (see below). Temperature-logging at a resol-
ution of 0.4 8C early in the project was improved
to 0.1 8C resolution at some sites from 2007. Spot
measurements were made at the time of sampling
at air and drip water sampling sites and all tempera-
ture measurements were corrected relative to a BS
certified mercury thermometer to an accuracy of
+0.05 8C.
Drip water was collected in a HDPE beaker contain-
ing an acoustic drip rate logger (Collister & Mattey
2008) and fitted with an outlet tube leading to a 1.5 l
storage reservoir open to cave air. The drip logger
counted the total drips falling in 30 minute intervals
and counts were converted to discharge in l/d based
on a mean drop volume of 0.15 ml (Collister &
Mattey 2008) for all sites.
Aliquots of water were stored in 100 ml HDPE
doubly sealed bottles and pH, conductivity and alka-
linity were measured within two hours of sampling.
pH was determined using a Jenway pH and conduc-
tivity probe which was calibrated at pH 7 and 10
before the sampling session. Alkalinity was deter-
mined on site by titration against bromocresol green
using a Hach digital titrator. Dripwater cation ana-
lyses were determined by ICP-AES in the Depart-
ment of Earth Sciences, Royal Holloway with a
total precision of less than 5%. All isotopic analyses
in this study were carried out using GV Instruments
Multiflow - Isoprime systems at Royal Holloway.
Drip water d
13
C DIC was determined by acidifica-
tion of 0.5 ml of water with orthophosphoric acid
and equilibrating for 7 h at 40 8C. d
13
C values
were normalized to the V-PDB scale via a calibrated
sodium bicarbonate internal standard and have an
external reproducibility of better than 0.08‰.
Surveying and dye tracing
A line survey was carried out to accurately position
the map of NSM made by Shaw (1954) relative to
surface topography. This showed that the soil moni-
toring site is located almost directly over the cave
(Fig. 2) and allowed us to identify sites for injecting
Air sampling and analysis
Spot samples of background atmosphere, cave
air and soil gas were collected in 1 l or 3 l Tedlar
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