Page 344 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Case studies 14/321
This impact is modeled with no sensitivity to actual population Ofthose leaks, 50 percent would contaminate a surface water sup-
density differences or index sum differences along the line. A thresh- ply in Tier 3, IO percent in Tier 2. Additionally, 75 percent would
old spill size of 50 bhl is assumed, below which frequencies of fatality contaminate a ground water supply in Tier 3, 25 percent inTier 2.
or injury are assumed to be zero. Using the tier miles, these aggregate to a 100 percent chance for
Further discussion of the fatality and injury rates used can be about 3 1 miles, or about 4 percent for the overall pipeline.
found in Attachment C ofthis report [Appendix F ofthis book]. The industry average leak rate applied to this pipeline predicts 35
leaks and, hence, about 6 spills (I6 percent of 35) would he of suffi-
cient volume to contammate a drinking water supply, and 0.2 spills
Drinking water contamination would occur at a location that contaminates a drinking water supply.
This is equivalent to saying one contamination episode occurs every
Drinking water contamination is defined as a potential level of con- five pipeline lifetimes or 250 years, since the 0.2 is based on a 50-
tamination which : year period.
causes an exceedance ofTexas dnnking water standards, or causes Index sum averages for each tier are used to estimate leak incident
an exceedance of proposedlexas ground water contamination lim- rates in Case 4. Further discussion of how this receptor is modeled can
its; and be found in Attachment C of this report [Appendix F of this hook].
can potentially impact a public drinking water supply for a period
of time exceeding normal system storage capacity (estimated at
about 24 hours). Drinking Water Contamination-No MTBE
The drinking water probability is a sum of the probability of impact- The previous impact assumes 15 percent MTBE is transported in the
ing ground water resources used for public drinking water supplies, pipeline. If no MTBE is present, the potential for impacts is assumed
and the probability of impacting surface water resources used for to he one-half of the previous case. Rationale for this is presented in
drinking water. Attachment C ofthis report [Appendix F ofthis book].
There are 29 miles along the pipeline rated sensitive or hypersensi- Edwards Aqu fer Contamination
tive for potential surface water drinking water quality. Based on sur-
face water modeling performed at the most hypersensitive locations This is a special case of“ground water drinking water contamination.”
long the pipeline, a threshold spill size of 1,500 hhl was set for surface focused specifically on the three miles between Milepost (MP) 170.5
water drinking water impacts. A spill smaller than this would not and MP 173.5 (all new pipe as proposed in LMP). Because ofthe doc-
(because of losses of water contaminants through natural processes umented pathways for rapid contamination of drinking water wells in
such as volatilization), pose drinking water quality impact, even under Sunset Valley, this represents “worst case” probability for ground
adverse climate (rainfall. evaporation) conditions. water contamination. This case has the following assumptions in addi-
tion to the general drinking water impacts.
There are 66 miles rated sensitwe or hypersensitive for poten-
tial ground water drinking water impacts. (Note: surface water Since this area is over known hypersensitive karst, the spill size
and ground water sensitive areas are not necessarily mutually threshold is set at 500 bbl. Spills of this size and larger are assumed
exclusive.) Based on the potential for various factors to retard trans- to he equally harmful.
port of contaminants to an aquifer, two separate threshold levels 0 In the mitigated case, the enhanced leak detection system in this
are set: area is credited with reducing the frequency of larger sized spills.
Specifically, the types of potentlal large spills reduced are those
Over porous media aquifers, confined or unconfined, a threshold of created by a slow leak, below the detection capabilities of normal
1.500 bbl reflects the potential for soil to ahsorh contaminants, and leak detection, continuing for long periods of time.
for conventional ground water remediation technologies such as 0 The index sum represents the additional leak prevention measures
pump-and-treat to control contaminants from reaching sensitive proposed in these three miles.
receptors.
0 Over hypersensitive karst aquifers, a lower threshold of 500 bbl Further discussion of how this receptor is modeled can he found in
reflects the potential for adsorption on the thinner soil layers over- Attachment C ofthis report [Appendix F of this book].
laying karst, and the rapid transport in karst aquifers which can
limit remediation effectiveness. Rose (Rose, 1986) [this reference
not included in this book] estimated this threshold at 1,000 bbl, Lake travis drinking water contamination
and a figure one-half that estimate was used to add a factor of
conservatism. This is a special case of“surface water drinking water contamination”
which focuses on spills in the Pedemales watershed that could impact
This impact is modeled as being sensitive to tier location, index sum, drinking water supplies drawn from Lake Travis. The potential for
and spill volume. Since the tier designations consider vulnerability of contamination of Lake Travis was analyzed in detail because of the
drinking water sources, a “probability of contamination” is assigned large number of people served by this reservoir (up to a million), and
for each tier. Depending on the vulnerability of a given resource, the duration contaminant levels in excess of drinking water cnteria or
threshold spill size is assumed before any impact is possible. Above advisory levels could he exceeded (on the order of I to 2 months for
that threshold, impacts are judged to he equally likely, regardless of any lake water users, including the City of Austin). The analysis
spilled volume. This is conservative, since even the spill volumes involves 1.54 miles of pipeline located in Tier 2 areas and 2.74 miles
closer to the threshold are modeled as being as harmful as the largest inTier 3. This represents worst case probability for contamination of
spill volumes. surface water used as a drinking water supply. The spill size threshold
is set at 1,500 bbl. Spills of this size and larger are assumed to he
An example of this impact is Case I shown in Table 3 and can be equally harmful and spills helow this threshold would not cause the
generally described as follows: impact.
I. About I6 percent of reportable leaks are of a size to pose a threat to Further discussion of how this receptor is modeled can be found
a drinking water supply inAttachment C ofthis report [Appendix F ofthis hook].