The 2001 crash plume consists mainly of PFHxS and PFOS (charts
in table row 1 below) whereas the area southwest of Del Ray
Avenue has significant amounts of these compounts but also has a
large amount of PFOA (charts in row 4 below). The source of the
additional PFOA is apparently the terminal apron (charts in row
5).
The main plume from the 2001 crash has a parallel ‘shadow’
plume just to its southwest, as depicted in
20210407_43_Interim_SIR.pdf PDF file p. 79 Fig 27 (below). Its
composition is depicted in the charts in table row 2 above. Like
the composition in the Del Ray Ave area, it is a mix of the
contributions from the 2001 crash site plus the terminal apron,
with a greater emphasis on the crash site than that seen in the
Del Ray Ave area.
The area near the 1970 crash site also appears to receive a
significant contribution from the high-PFOA terminal apron (see
chart table row 3 above).
The composition of the terminal apron PFAS deserves special
comment. According to ITRC
“Legacy fluorotelomer AFFF were manufactured and sold in the
United States from the 1970s until 2016 and encompass all other
brands of AFFF besides 3M Light Water or their licensed products
(Schultz et al. 2006[57]). Although they are not made with PFOA,
they contain polyfluorinated precursors (Backe, Day, and Field
2013[12]) (Place and Field 2012[312]) that are known to degrade
to PFCAs, including PFOA (Weiner et al. 2013[882])
(Harding-Marjanovic et al. 2015[304]).”
The terminal apron results also show significant levels of 6:2
FTS which suggests the additional presence of newer C6
fluorotelomers which also degrade to PFCAs, including PFBA but
not PFOA. The terminal apron is the likely source zone for the
PFBA which appeared at significant levels in the Del Ray Ave
area, to a lesser degree in the main residential plume from the
2001 crash, and at intermediate levels in the ‘shadow’ plume to
its southwest. PFBA acts as a tracer and at significant
distances from the source zone tends to overwhelm the other
analytes, so I followed common practice by removing it from the
charts in order to make them easier to interpret.
6:2 FTS is mobile in the groundwater. It’s presence in the
source zone but not elsewhere could be partially due to the
extra exposure to oxygen in standing water that accumulates in
the western infiltration area during heavy runoff events.
I grouped the private well data by using the maps from
20210804_43_PW_Sample_Results.pdf Fig 3c (Del Ray Ave), Fig 3d
and Fig 3b.
2001 crash main plume extending to Dawson Ave interchange
includes addresses on :
Bainbridge St samples 338, 341, 342 (also 343 on 1st Ave
West), 110-2, 110-1, 111, 229, 230
1st Ave East; 251, 252, 255, 256, 197, 200, 202
2nd Ave East 214, 215, 218,
219, 220, 186, 188, 189
2001 crash SW ‘shadow’ plume has addresses on:
1st Ave West: samples 384, 385, 386, 387, 388, 389, 328,
332, 335
Bainbridge: 329, 330
Del Ray Ave has addresses on:
Del Ray (east side): samples 84,
88, 89, 91, 94, 95, 96
Del Ray (west side): 79,
1049, 1050, 1051
Grand Ave (east side): 83
Grand Ave (west side): 47, 49, 72,
1047, 1048, 1193
11-1, 27, 30, 412, 1045
samples 72-0 2713 Grand Street and 255-0 2545 1st Ave E
have multiple problems so they were dropped from the analysis
Lake Shore Drive near 1970 crash site has samples:
1004, 1016, 1024, 1027, 1425, 1433, 1435, 1437, 1438, 2317
Most of the information regarding water movement in the airport
is in a stormwater management memo.
20210407_43_Interim_SIR.pdf PDF p. 119 Attachment D Airport
Stormwater Drainage Memo 3/11/2021
PDF file p. 122 contains Fig 2: Airport Overview - South
Airport
As mentioned in the summary there are two areas in the
southern part of the airport where runoff infiltrates:
(1) West infiltration area: serving the area west of the
main runway and clearly visible as a dark oval between the Del
Ray Avenue area to the west and the junction of the main
north/south and NE/SW runways to the east.
(2) East infiltration area: serving the area east of the
main runway near the intersection of Fanta Reed Rd and Dawson
Ave, indicated by the small red arrow pointing NNE from Fanta
Reed Road.
The west infiltration area receives runoff from the terminal
apron which mostly drains to the grassy area along its eastern
edge. Some of the runoff passes through culverts under the main
taxiways serving the apron. It apparently also receives runoff
from the storm sewer under Airport Road. Groundwater mounding
would tend to push some of the contamination westward toward Del
Ray Ave.
There is a third infiltration area at the north end of the
airport served by the same Airport Road storm sewer seen in Fig
2 above.
It is described by the interim site investigation report:
14.2.1.2 Terminal, Parking & Airport Drive
Stormwater runoff on Airport Drive, the terminal and
associated parking area either infiltrates to the groundwater
or is carried by storm sewer to a large stormwater retention
and settling pond, located at the north end of the airport,
adjacent to the bay formed by the extension of Runway 18 to
Bell Island. The pond, in turn, discharges at an outfall into
the bay. Lenz noted, that during high flow conditions a
portion of the stormwater will discharge via a storm sewer
pipe, just south of the terminal, into the internally drained
airfield.
Although Airport Drive, the terminal and associated parking
areas are not considered potential source areas, should
stormwater in these areas contain PFAS, it would infiltrate to
the groundwater or be carried by storm sewer north to the
stormwater retention pond and then to the bay formed by the
extension of Runway 18 to Bell Island. In the event of leakage
from the storm sewer, PFAS contamination would reach the
groundwater flow which is flowing southeasterly in that
location.
I have not seen evidence of any attempt to sample either the
west infiltration area which receives runoff directly from the
terminal apron or the northern stormwater retention pond which
receives water from an area at the southwest corner of the
terminal apron. This is despite their being mentioned as
potential source areas in the interim site investigation report.
9.2.7 Areas of Stormwater
Infiltration
Areas of stormwater
infiltration for the internally drained portions of the
airport, particularly adjacent to areas of frequent or heavy
airport and tenant operations, are considered potential
source areas. See the attached March
16, 2021 memorandum regarding Airport Stormwater Drainage21 from Bernard Lenz, P.E. – La Crosse
Utility Manager.
Indeed, the interim report only attempts to explain the spread
of high-PFOA contamination by referring to background levels in
the adjoining river:
Based on concentrations observed in up-gradient wells along
Lake Onalaska, it is believed that at least a portion of the
PFAS detections can be attributed to “background” levels from
Lake Onalaska/Mississippi River in groundwater recharge of the
sand-and-gravel aquifer.
Potential discharge of relatively high-level PFAS contamination
from the storm sewer is especially concerning given that its
outfall goes directly to a major groundwater recharge area for
the island, seen as a red band along the northern portion of the
island in Fig 11b from the interim site investigation report.
The east infiltration area receives runoff from a storm sewer
that runs lengthwise under the large SW/NE apron in front of the
fire station. Zooming in on Google Maps satellite view shows
dark circles along this line, presumably sewer grates. It
apparently dumps out to a culvert opening visible in the mottled
area where the apron bends to north/south. In the drainage map
above see the tip of the large red arrow showing the direction
of flow through the apron storm sewer.
Surface flow proceeds south entering a culvert under the
taxiway which serves the south end of the main runway. Outflow
from this culvert (along with that from another serving the
grassy area adjoining the runway intersection) enters a narrow
dark strip leading SSW to a low area stretching NNW/SSE toward
Fanta Reed Road just west of its intersection with Fanta Reed
Pl. In the drainage map above the small red arrow shows the
direction of flow through a culvert under the road which empties
into the low area.
Several culverts under this part of Fanta Reed road appear to
align with residential streets. It is not immediately clear
whether this infiltration area is also receiving runoff from the
pavement in the residential develpment south of the road.
However, the USGS topographic viewer indicates a drop of 5
meters as you proceed NE across this development toward the
infiltration area.
Mounding in this area would tend to push contamination from
the 2001 crash toward the WNW. This would explain the sulfonates
in the Del Ray Ave area as well as the composition of the
‘shadow’ plume SW of the main plume from the 2001 crash site.
Lee Donohue provided photos taken 5/5/2023 of water standing
in this area. She commented: “This water has been there since
last week although we've barely had rain in a week.” It was
still there in subsequent photos taken on 5/18.
I suspect that elevation of the water table due to the spring
flood – the highest since 2001 – has effectively reduced the
gradient of the groundwater mound and slowed lateral movement,
causing it to take longer than usual for the surface water to
recede. According to the Minnesota
Stormwater Manual, a shallow water table is a risk factor
for mounding:
-
Thin vadose zone. Mounding is more likely to occur in
areas with a shallow groundwater table (thin vadose zone).
There
is strong evidence in the PFAS sampling results that transport
is much more complex than the groundwater model suggests. The
sampling results map shows that in the Del Ray Avenue area
contamination levels are highest near the airport boundary and
decline with distance to the west.
_2023-05-21.png)