Updated October 20, 2026
Short answer
Almost all of it, on the public maps. Quick Flow Septic checked 4,216 sample points around 168 towns in 15 Houston-area counties against the USDA soil survey, FEMA's flood maps and the National Wetlands Inventory. The USDA rates 96% of the points that carry a soil rating "very limited" for a conventional septic drainfield, and the land check's mapping to the Texas septic rules sends 99% of them to an aerobic system instead; 22% of the points on land sit in a FEMA high-risk flood zone. Only the two soil borings on your own lot decide what you can install.
What the study found
Of the 4,052 sample points with a USDA soil rating, 3,907 (96%) are rated "very limited" for septic tank absorption fields. That is the most restrictive of the USDA's three ratings: soil with at least one problem that generally can't be overcome without special design or expensive installation. Run through the same mapping our free land check uses, 4,016 of those points (99%) point to an aerobic treatment unit instead of a conventional gravity drainfield: 77% to aerobic with spray dispersal and 22% to aerobic with drip. Only 36 points (1%) mapped to a conventional system.
How the counties compare
Every county in the study is mostly "very limited" ground, from 89% of rated points around San Jacinto County towns to 100% around Walker County towns. Of the 36 points that mapped to a conventional system, 15 are around San Jacinto County towns, the most of any county. Each county name below links to its page, with the county's permit office, the permit rules and the towns we serve there.
Septic soil results by county
| County | Towns · sample points | Rated "very limited" | Point to an aerobic system | Main limiting factors |
|---|---|---|---|---|
| Austin County | 13 towns · 325 points | 95% (307 of 322) | 100% (322 of 322) | Slow drainage (89%) · High water table (25%) |
| Brazoria County | 5 towns · 110 points | 96% (105 of 109) | 100% (109 of 109) | Slow drainage (98%) · Ponding (54%) |
| Chambers County | 5 towns · 110 points | 96% (72 of 75) | 100% (75 of 75) | Slow drainage (100%) · High water table (93%) |
| Colorado County | 12 towns · 290 points | 94% (267 of 284) | 97% (276 of 284) | Slow drainage (85%) · Ponding (32%) |
| Fort Bend County | 19 towns · 483 points | 95% (444 of 465) | >99% (463 of 465) | Slow drainage (94%) · Ponding (61%) |
| Grimes County | 11 towns · 268 points | 98% (257 of 263) | >99% (262 of 263) | Slow drainage (99%) · High water table (42%) |
| Harris County | 14 towns · 412 points | 99% (384 of 388) | 100% (388 of 388) | Slow drainage (95%) · High water table (82%) |
| Liberty County | 17 towns · 413 points | 99% (405 of 408) | >99% (406 of 408) | Slow drainage (92%) · High water table (88%) |
| Montgomery County | 18 towns · 487 points | 98% (464 of 474) | 100% (474 of 474) | Slow drainage (90%) · High water table (77%) |
| Polk County | 5 towns · 110 points | 89% (84 of 94) | 91% (86 of 94) | Slow drainage (64%) · High water table (60%) |
| San Jacinto County | 10 towns · 233 points | 89% (182 of 204) | 93% (189 of 204) | Slow drainage (75%) · High water table (67%) |
| Walker County | 6 towns · 158 points | 100% (154 of 154) | 100% (154 of 154) | Slow drainage (100%) · High water table (73%) |
| Waller County | 11 towns · 268 points | 96% (256 of 266) | 100% (266 of 266) | Slow drainage (90%) · High water table (57%) |
| Washington County | 10 towns · 233 points | 95% (219 of 230) | 100% (230 of 230) | Slow drainage (92%) · Shallow bedrock (16%) |
| Wharton County | 12 towns · 316 points | 97% (307 of 316) | 100% (316 of 316) | Slow drainage (>99%) · Ponding (61%) |
| All sampled counties | 168 towns · 4,216 points | 96% (3,907 of 4,052) | 99% (4,016 of 4,052) | Slow drainage (91%) · High water table (54%) |
Why the ground rules out a standard drainfield
Slow drainage is the main reason. 91% of the rated points show water moving through the soil too slowly: clay, or soil the USDA places in its slowest-draining hydrologic group. 54% show groundwater close to the surface in wet months, 31% show water ponding after rain, and 24% carry a flooding hazard; most points have more than one. A conventional drainfield relies on the soil to soak up the wastewater and filter it before it reaches groundwater, so any of these moves the design to an aerobic unit, which treats the water first and then spreads it by spray or drip. The most common soil series in the sample are Lake Charles, Edna, and Brazoria.
Flood zones and wetlands
22% of the sample points on land (909 of 4,063) fall inside a FEMA Special Flood Hazard Area, the high-risk 100-year floodplain, from 10% around Washington County towns to 35% around Wharton County towns. 189 sit in a regulatory floodway, where the land check rules out spray dispersal. A lot in the floodplain adds floodplain rules to the septic design and usually a county floodplain permit. 20% of land points (802 of 4,042) have a wetland mapped by the U.S. Fish and Wildlife Service within about 100 feet, the most around Liberty County towns (42%); a wetland can shrink the room for a drainfield or spray area, and the land check sends those points to drip dispersal.
Flood zones, wetlands and soils by county
| County | In a FEMA high-risk flood zone | Wetland mapped within ~100 ft | Most common soil series |
|---|---|---|---|
| Austin County | 20% (63 of 322) | 17% (54 of 322) | Frelsburg, Lake Charles, Tremona |
| Brazoria County | 34% (37 of 109) | 23% (25 of 109) | Lake Charles, Bernard, Edna |
| Chambers County | 32% (24 of 76) | 33% (25 of 76) | Morey, League, Vamont |
| Colorado County | 22% (62 of 284) | 11% (32 of 284) | Straber, Nada, Tremona |
| Fort Bend County | 27% (127 of 466) | 15% (69 of 466) | Lake Charles, Edna, Bernard |
| Grimes County | 14% (37 of 263) | 17% (45 of 263) | Shiro, Falba, Frelsburg |
| Harris County | 17% (68 of 390) | 23% (87 of 384) | Wockley, Clodine, Labelle |
| Liberty County | 35% (142 of 409) | 42% (172 of 409) | Kaman, Evadale, Beaumont |
| Montgomery County | 22% (104 of 480) | 20% (98 of 480) | Sorter, Conroe, Splendora |
| Polk County | 14% (13 of 94) | 10% (9 of 94) | Pinetucky, Woodville, Hatliff |
| San Jacinto County | 14% (29 of 204) | 15% (31 of 204) | Woodville, Hatliff, Pinetucky |
| Walker County | 11% (17 of 154) | 18% (28 of 154) | Annona, Depcor, Falba |
| Waller County | 19% (51 of 266) | 22% (58 of 266) | Wockley, Katy, Hockley |
| Washington County | 10% (23 of 230) | 13% (29 of 224) | Frelsburg, Bleiblerville, Latium |
| Wharton County | 35% (112 of 316) | 13% (40 of 307) | Lake Charles, Edna, Brazoria |
| All sampled counties | 22% (909 of 4,063) | 20% (802 of 4,042) | Lake Charles, Edna, Brazoria |
What it means if you're buying land
Budget for an aerobic system unless the soil borings say otherwise, and check the flood zone before you close. Quick Flow Septic's flat installed prices are $8,000 for a conventional system, $11,950 for aerobic with spray and $13,950 for aerobic with drip. They cover the system and its installation; the soil evaluation, the engineered design, the county permit, and the aerobic maintenance contract are quoted separately. For one specific lot, run its address through the free land check: the same soil, flood and wetland lookups as this study, on that exact point, in about ten seconds.
Check a lot's address for freeHow we did it
We sampled points, not whole counties. Around each town we serve we laid a grid of points: 1.6 miles apart out to 5 miles from the center of the 40 towns that have their own page on this site, and 1.2 miles apart out to 3 miles for 128 smaller communities. The center is the U.S. Geological Survey's place point for the town; 3 communities without one weren't sampled. Every point went through the same lookups as our free land check between October 2 and October 20, 2026: the USDA NRCS soil survey (SSURGO, via Soil Data Access) rating for septic tank absorption fields and its limiting reasons; the flood zone from Esri's FEMA-derived copy of the National Flood Hazard Layer (the land check's fallback source); and the USFWS National Wetlands Inventory within about 100 feet. The land check then maps those signals to the system the Texas septic rules (30 TAC Chapter 285) would likely call for. A "very limited" rating, slow-draining or clay soil, a water table within about 3 feet, ponding, flooding, bedrock within 2 feet, or fast seepage rules out a conventional drainfield; a floodway, a slope over 15%, or a wetland on site points to drip. The study covers all 15 counties we serve.
What the study can't tell you
A map sample isn't your lot. The USDA soil survey maps the landscape at 1:24,000, so a single tract can differ from the map unit it sits in, and Texas requires a licensed site evaluator or professional engineer to evaluate every tract, with two soil borings, before a permit is issued (30 TAC §285.30). The figures are shares of sample points, not of land area: circles around neighboring towns overlap, so some ground is counted twice, and a circle near a county line can reach into the next county, with each point counted under the county of the town it surrounds. The 153 points that fall on open water (lakes, rivers, bays) are left out of every share, since water sits in the flood zone and on the wetland map by definition; points on pits or landfills carry no septic rating and are left out of the soil and system shares. City sewer, lot size and setbacks are not part of the sample.
How to cite this study
You're welcome to quote and reuse these figures: they're published under a Creative Commons Attribution 4.0 license (CC BY 4.0), so please credit Quick Flow Septic and link to this page. Suggested citation: Quick Flow Septic, "How Much Land Around Houston Can't Take a Conventional Septic System?" (October 2026), https://quickflowseptic.com/guides/houston-septic-soil-study. Key finding: the USDA soil survey rates 96% of 4,052 rated sample points around 168 Houston-area towns "very limited" for a conventional septic drainfield (data checked between October 2 and October 20, 2026).
Free tool
Will your lot need an aerobic system? Check the address.
USDA soil rating, FEMA flood zone, and mapped wetlands for any Texas address — plus the system type and flat price you're likely looking at. About ten seconds, no account.
Frequently asked questions
Can you put a conventional septic system on clay soil in Texas?
Usually not. Under 30 TAC Chapter 285, heavy clay (Class IV soil) rules out a standard gravity drainfield, and the design moves to an aerobic unit with spray or drip dispersal. In our study, 91% of the rated points show water moving through the soil too slowly. The site evaluator's two soil borings decide the soil class on your lot.
What does "very limited" mean in the USDA soil survey?
It's the most restrictive of the USDA's three ratings for septic tank absorption fields (not limited, somewhat limited, very limited). It means the soil has at least one feature, such as slow drainage, a high water table, flooding, ponding, shallow bedrock or fast seepage, that generally can't be overcome without special design or expensive installation. In Texas that usually means an aerobic system with spray or drip instead of a gravity drainfield.
Which county had the most land that could take a conventional system?
San Jacinto County: 15 of the 204 rated points around its towns (7%) mapped to a conventional system, the most of any county in the study. Across the whole study, 36 of 4,052 rated points did (1%).
How much land around Houston is in a FEMA flood zone?
In our sample, 22% of 4,063 points on land fall in a FEMA high-risk flood zone (Special Flood Hazard Area), from 10% around Washington County towns to 35% around Wharton County towns. A septic system in the floodplain needs floodplain-aware design and usually a county floodplain permit.
Does this mean my lot needs an aerobic system?
Not necessarily. The USDA maps soil at landscape scale (1:24,000), and every Texas tract gets its own site evaluation with two soil borings before a permit (30 TAC §285.30). Use the free land check for a first read on your address; the borings make the final call.
What does an aerobic system cost compared with a conventional one?
At Quick Flow Septic, a conventional system is $8,000 installed, aerobic with spray $11,950, and aerobic with drip $13,950. Those prices cover the system and its installation; the soil evaluation, the engineered design, the county permit, and the aerobic maintenance contract are quoted separately.
Where does the data come from?
Public federal data: the USDA NRCS soil survey (SSURGO, via Soil Data Access), FEMA's flood hazard maps and the USFWS National Wetlands Inventory, read for each point between October 2 and October 20, 2026 through our land check, which maps them to the Texas septic rules (30 TAC Chapter 285).
Can I use these numbers in an article or a listing?
Yes. Please credit Quick Flow Septic and link to this page; the suggested citation is in the "How to cite this study" section above.
Quick Flow Septic installs new septic systems across Harris County · Montgomery County · Fort Bend County · Waller County · Liberty County · Washington County · Grimes County · San Jacinto County · Walker County · Colorado County · Austin County · Wharton County · Brazoria County · Chambers County · Polk County.
Ready for a real number?
Get your instant estimate, or call and we'll schedule a free on-site evaluation.