OVERVIEW OF REGIONAL GEOLOGY AND HYDROGEOLOGY, AND SUBSURFACE INVESTIGATION METHODS:
CONSIDERATIONS FOR MSW LANDFILLS IN TEXAS
Presented by: Terry Virtue and Eric CleggWaste Permits Division
2018 TCEQ Environmental Trade Fair
Photograph from AGS Website (William R. Muehlberger)
AGENDATerry Virtue
1. Geology regions of Texas
a) Gulf Coast
b) Llano Uplift
c) North Central Texas
d) Panhandle
e) West Texas
2. Texas Aquifers
Eric Clegg1. MSW Geology Report
2. Subsurface Investigation
3. Interpretation of Field Data
photo from USGS.
LimestoneLimestone
MJJ
Clay
Gneiss
Sandstone
Granite
Volcanics
Sand
Siltstone
Shale
Sand Schist
GULF COAST
30 TAC 584(b)(4): 1,000 feet protected and 5,000 feet not protected.
Shrink/Swell Clays
330.547: Flooding
Shallow Groundwater
Channel
Sediments
Beach
Barrier IslandBay
photo from texascoastgeology.com
FAULTING AND SUBSIDENCE
Faulting caused by subsidence.
Groundwater withdrawal & recent subsidence.
200 foot buffer & ½ mile rule for active Faults: 30 TAC 330.555
Edwards Aquifer: 30 TAC 330.549 (Chapter 213).
Unstable areas are restricted according to 30 TAC 330.559.
Balcones Fault Zone Hidden Valley Fault scarp
LLANO UPLIFT
photos courtesy of Terry Virtue
Enchanted Rock
Town Mountain Granite
Packsaddle Schist
Lack of natural clays for liner and cover requirements.
Hard rock makes excavation difficult.
NORTH CENTRAL TEXAS
Pennsylvanian (blue), Permian (light blue) and Cretaceous rocks (greens) north of the Llano uplift.
Most landfills in impermeable limestones and shale.
Smithwick Formation
photo from Austin Geological Society website.
PANHANDLE
Caprock canyon photo from Caprock Canyon State Park website. Wink Sinkhole photo from beg.utexas.edu.
WEST TEXAS
Chisos Mountainsphoto from Brian Hunt, AGS
SEISMIC HAZARDS
Area >10% probability of >.10 g in 250 years
Seismic Impact Zones: 30 TAC 330.557
Seismic hazard map from USGS
FENCE DIAGRAMLack of clay for cover
and liner systems.
Deep groundwater beyond 200 feet depth in Quaternary Alluvium.
Landfills located mostly in alluvial deposits.
AQUIFER CONSIDERATIONS
Class I cell rules: 30 TAC 335.584(b)(1)
Taylor Clay
Terrace Deposits
GEOLOGY REQUIREMENTS FOR MSW FACILITIES
Includes: Description of regional geology,
faults and subsidence, and groundwater resources/regional aquifers from research
The results of a subsurface investigation that describes all borings logged using USCS descriptors.
Cross-sections prepared from the logs.
A narrative that describes the licensed professional's interpretations of the subsurface
GEOLOGY REPORT required by: 30 TAC §330.63(e) and §332.47(6)(B).
Photo courtesy of www.tourtexas.com
PRIMARY GEOLOGIC AND HYDROGEOLOGIC QUESTIONS TO BE ADDRESSEDA good permit application needs to:
Clearly define the uppermost aquifer, any underlying connected aquifers, and underlying aquitard (depths and thicknesses) in relation to the proposed landfill
Determine the direction of groundwater flow and any influence from lithology
Propose wells in the right places (Well spacing/ point of compliance (POC) vs. upgradient/background)
Determine if there is an uplift concern or if dewatering is required
THE SUBSURFACE INVESTIGATION STARTS WITH THE BORING PLAN. WHAT DOES A BORING PLAN REQUIRE?
Locations and depths of all proposed borings
The required number of borings per the table
All borings be at least 5 feet below the elevation of the deepest excavation (EDE) with a min. number of borings to a depth at least 30 feet below the EDE
Size of Area in Acres Number of Borings Min. No. of Borings 30 Ft below the EDE
5 or less 2‐4 2
5‐10 4‐6 3
10‐20 6‐10 5
20‐50 10‐15 7
50‐100 15‐20 7‐12
100‐150 20‐23 12‐13
150‐200 23‐26 13‐15
200‐250 26‐29 15‐16
250‐300 29‐32 16‐17
300‐350 32‐35 17‐18
350‐400 35‐38 18‐20
400‐450 38‐42 20‐21
450‐500 42‐44 21‐22
500‐550 44‐47 22‐24
550‐600 47‐50 24‐26
More than 600 Determined in consultation with the executive director
Note: Different for Arid Exempt (AE) landfills
EDE
Not the EDE
5’ > EDE
30’ > EDE
5’ > EDE
30’ > EDE
HOW DOES THE REGIONAL GEOLOGY AND AQUIFERS RELATE TO THE SUBSURFACE INVESTIGATION?
Regional geology will indicate most likely drilling method required.
Knowing the depth below grade and the thickness of a Texas Major or Minor Aquifer under your site can tell you whether they are shallow enough to be included in the subsurface investigation.
Edwards Aquifer recharge zone areas are prohibited from Type I landfills
DRILLING METHODS3 COMMON METHODS IN USE FOR ENVIRONMENTAL DRILLING
Hollow-Stem Auger Air/Mud Rotary Geoprobe/Direct Push
Courtesy of Keely and Boateng, 1987 Courtesy of geoprobe.com
METHOD COMPARISONMethod Advantages DisadvantagesHollow-Stem Auger (HSA) • Continuous sample collection
• Borehole walls can’t collapse in soft sediments
• Slow• Seams can get clogged in very
fine sands
Air/Mud Rotary • Can drill into hard bedrock• Provides blow counts to
determine sediment density (n-value)
• Slow• Boreholes can cave in while pipe is
out of the hole (Esp. with air)• Difficult to collect samples in
saturated zones
Geoprobe/Direct Push • Quick• Very mobile• Continuous sample collection• Also provide blow counts• Can use acetate sleeves to seal
& save cores
• Can’t drill through hard sediments/gravels/bedrock
• Typically can only drill to ~50’• Small 2” diameter borehole only
allows for small diameter wells
WHAT DOES THE FIELD GEOLOGIST DOCUMENT IN THE FIELD? They assess the core samples to determine and record
where breaks between different lithologies occur.
Use the Unified Soil Classification System (USCS) to describe the soils at set intervals (every 5’ to 10’) or whenever changes occur and record it on a boring log.
Keep track of:
drilling info (dates, times, location, rig type/model, size of borehole/augers);
depths (to water, bottom of hole, sample intervals, etc.); and
well materials used and depths to each component of wells, if constructed.
USCS CHART
USCS is useful for logging non-bedrock soils, first by overall grain size percentages, then gradation, plasticity, particle shape/roundness, moisture, density, color, etc.
Courtesy of W.F. McCollough
Gradation
Grain size distributionCOARSE-GRAINED DESCRIPTORS
COARSE-GRAINED DESCRIPTORS (CONT.)
Shape/RoundnessDegree of Compaction (using N-Value)
PlasticityDegree of Compaction (using N-Value)
FINE-GRAINED DESCRIPTORS
THEN THE FIELD DATA IS TURNED INTO A BORING LOG
THEN THE LOGS ARE TURNED INTO CROSS-SECTIONS
OTHER USES OF THE DATA…
Structure Contour Maps
Fence Diagrams
3D Computer Models
Courtesy of pubs.usgs.gov
Courtesy of British Geological Survey
World Journal of Environmental Engineering. Vol. 3, No. 3
THANK YOU!
Contact Information
Texas Commission on Environmental QualityP.O. Box 13087, Austin, Texas 78711
MSW Permits SectionTerry Virtue, P.G. Eric Clegg, P.G.
[email protected] [email protected]
512-239-3142 512-239-1270Photograph from Austin Geological Society website (Brian Hunt)
