Guest Opinion: Proposed new wells will spread PFAS plume

The Livermore/Amador (Tri Valley) groundwater basin contains both vertical and horizontal preferential flow pathways, including fractured zones, paleo channels, and high permeability sand/gravel lenses. These features can accelerate PFAS migration, create finger like plumes, and allow contaminants to move deeper or laterally faster than in a uniform aquifer. Zone 7’s own PFAS monitoring and modeling efforts acknowledge this complexity, which is why they’ve added sentinel wells and are upgrading their groundwater model to better track plume movement.
What we know about Tri Valley aquifer structure
The Livermore Valley Groundwater Basin is heterogeneous, built from:
• Alluvial fan deposits (coarse gravels → fast horizontal flow)
• Interbedded silts/clays (slow flow, but can create perched zones)
• Buried paleo channels (high permeability “pipes” that move water quickly)
• Fractured bedrock margins (vertical leakage pathways)
These act like vertical and horizontal “columns” or conduits, even though they aren’t literal columns — they’re geologic heterogeneities that create preferential flow.
Zone 7’s groundwater modeling updates and sentinel well installation confirm that PFAS plume movement is influenced by these preferential pathways, requiring more detailed tracking.
How this affects PFAS plume behavior
PFAS compounds (especially PFOS/PFOA) are:
• Highly mobile in groundwater
• Persistent (don’t degrade naturally)
• Weakly sorbing (don’t stick strongly to soil)
Because of that, the aquifer’s structure matters a lot.
1. Horizontal high permeability zones → faster lateral plume spread
Coarse sand/gravel layers and paleo channels can:
• Carry PFAS farther from the source
• Create elongated plumes
• Connect wells that otherwise wouldn’t influence each other.
2. Vertical pathways → deeper contamination
Fractures, faults, and coarse vertical lenses can:
• Allow PFAS to migrate downward into deeper pumping zones.
• Connect shallow contamination to municipal well screens.
• Make plume boundaries harder to predict.
3. Pumping from Pleasanton & Zone 7 wells can distort plume shape
When wells pump:
• They create cones of depression.
• These can pull PFAS toward the wellfield, even from distant zones.
• Preferential pathways amplify this effect.
Zone 7 has already reduced pumping in wells where PFAS was detected and added sentinel wells to track plume movement.
Why Zone 7 is investing heavily in PFAS treatment
Because of the aquifer’s complex flow pathways, Zone 7:
• Built PFAS treatment plants at Stoneridge and Chain of Lakes
• Is designing a third at Mocho Wellfield
• Continues to monitor and model plume movement.
• Uses sentinel wells to detect shifts in PFAS concentrations early.
This is exactly the kind of response you’d expect when an aquifer has non uniform flow pathways that make plume behavior less predictable.
• Likely PFAS migration routes in the Tri Valley basin
• How specific wells (Mocho, Stoneridge, Bernal, Pleasanton) interact with the plume.
• What geologic units are most responsible for vertical vs. horizontal movement?
• How pumping changes plume geometry over time.
A diagram-style explanation, a plain language summary, or a technical hydrogeologic breakdown?

1. Likely PFAS migration routes in the Tri Valley basin
Big picture: PFAS in the Livermore Valley Groundwater Basin tends to move:
• From source areas (industrial/commercial zones, historical fire training/foam use, landfills, etc.)
• Into shallow alluvial groundwater (sands/gravels)
• Then laterally along higher permeability layers and downward where vertical pathways exist.
• Toward major pumping centers: Mocho Wellfield, Stoneridge, Chain of Lakes, Pleasanton wells
Because PFAS is highly mobile and persistent, once it’s in the aquifer, it tends to form elongated plumes that follow groundwater flow and any “fast lanes” in the geology (coarse channels, fractures).
2. How specific wells interact with the PFAS plume
Mocho Wellfield (Pleasanton)
• Role: Major groundwater producer; now known to have PFAS in local groundwater.
• What’s happening:
o Zone 7 has reduced Mocho production by about two thirds to limit PFAS affected pumping.
o A Mocho PFAS Treatment Plant is being designed to restore wellfield capacity while removing PFAS via ion exchange.
• Plume interaction:
o Historically, strong pumping here would have pulled PFAS bearing water toward Mocho, potentially deepening and elongating the plume.
o Reduced pumping now lessens the hydraulic pull, which can slow plume advance toward these wells but doesn’t remove PFAS already in the aquifer.
Stoneridge wells
• Role: One of the first PFAS impacted wellfields to get treatment.
• What’s happening:
o The Stoneridge IX PFAS Treatment Plant (online since 2023) treats groundwater before it enters the distribution system.
• Plume interaction:
o With treatment in place, Zone 7 can continue pumping without sending PFAS to customers, but pumping still influences plume shape, drawing PFAS bearing water along preferential pathways toward the well screens.
Chain of Lakes wellfield
• Role: Large groundwater production area with a major PFAS treatment facility.
• What’s happening:
o The Chain of Lakes PFAS Treatment Facility (largest IX plant of its kind in Northern California) treats up to 10 MGD of groundwater.
• Plume interaction:
o High capacity pumping here can reshape regional flow, potentially capturing PFAS plumes that migrate from upgradient sources and helping “contain” them hydraulically—though the PFAS mass still has to be removed at the plant.
Other Pleasanton/municipal wells
• Role: Some are directly operated by Zone 7, others by local retailers (Pleasanton, Dublin, Livermore).
• Plume interaction:
o Any well screened in PFAS impacted zones can act as a sink, pulling the plume toward it.
o Operational changes (turning wells on/off, changing pumping rates) can shift plume direction and concentration patterns over time.
3. Geologic units controlling vertical vs. horizontal movement
Think of the aquifer as layered and patchy, not uniform.
• Horizontal “fast lanes”:
o Coarse sand and gravel lenses in alluvial fan deposits
o Buried paleo channels (old river channels filled with permeable material)
o These act like horizontal conduits, letting PFAS move farther and faster laterally.
• Vertical pathways:
o Fractures and faults near basin margins or in consolidated sediments
o Coarse vertical or cross cutting lenses that connect shallow and deeper zones.
o Leaky confining layers where clays are discontinuous.
o These allow PFAS to leak downward from shallow contamination into deeper production zones.
• Retarding layers:
o Silt and clay beds slow flow and can temporarily “hold” PFAS in pore water, but because PFAS is persistent, this is more of a delay than a permanent barrier.
So, when you say “vertical and horizontal columns,” the real world analogs are these preferential pathways—zones of higher permeability that behave like subsurface pipes and chimneys.
4. How pumping changes PFAS plume geometry over time
Pumping does not just remove water—it re sculpts the pressure field in the aquifer.
• Cones of depression:
o Each pumping well creates a cone of lowered water levels.
o PFAS bearing water flows downhill into that cone, bending the plume toward the well.
• Plume stretching and narrowing:
o Strong pumping along a line of wells (e.g., a wellfield) can stretch the plume in the direction of flow and narrow it perpendicular to flow, like pulling taffy.
• Vertical capture:
o Deep wells can pull PFAS downward from shallower zones if vertical pathways exist, effectively connecting shallow contamination to deeper screens.
• Operational changes:
o When Zone 7 reduces pumping at Mocho and increases reliance on treated wells and surface water, the plume’s center of mass and direction of travel can shift.
o Over years, this can mean new areas become more impacted while others stabilize or slowly improve.
———————- Deeper aquifer (production zones) ————————
→→→ PFAS drawn downward and laterally into well screens
You can imagine horizontal “pipes” in the shallow alluvium and vertical “chimneys” where fractures or coarse lenses cut through finer layers. Pumping wells at depth pull water from both, shaping the PFAS plume.
You are asking really sharp questions here—this is the kind of thing hydrogeologists argue about in conference hallways. I will keep it clear but not dumbed down.
1. Likely PFAS migration routes in the Tri Valley (Livermore–Pleasanton) basin
Conceptual flow pattern (big picture):
• Recharge areas: Upgradient alluvial fans and valley margins (Livermore side, Arroyo Mocho/Arroyo Valle areas) recharge the basin.
• Regional flow direction: Groundwater generally moves from east/northeast (Livermore) toward west/southwest (Pleasanton/Bernal/Chain of Lakes), following the valley axis and arroyos.
• PFAS entry points (likely):
o Historical/ongoing industrial/commercial areas
o Fire fighting foam use (training sites, airports, industrial fire protection)
o Landfills or waste handling sites
• PFAS migration path:
1. Infiltrates to shallow alluvial aquifer (sands/gravels).
2. Moves laterally along higher permeability layers (paleo channels, coarse fan deposits).
3. Leakes downward where vertical pathways exist (fractures, coarse lenses, leaky clays).
4. Is drawn toward pumping centers: Mocho, Stoneridge, Chain of Lakes, Pleasanton wells.
So the PFAS plume does not move as a smooth oval—it follows “fast lanes” in the subsurface and bends toward where water is being pumped.
2. How key wellfields interact with the PFAS plume
Mocho Wellfield (Pleasanton area)
• Hydro role: Major production zone tapping the alluvial aquifer near Arroyo Mocho.
• PFAS interaction:
o If PFAS is present upgradient or in shallow zones, Mocho’s pumping pulls it laterally and vertically toward the screened intervals.
o Historically, strong pumping would have deepened the cone of depression, increasing vertical leakage from shallow PFAS bearing water.
o Reducing pumping (as has been done in PFAS affected wells) shrinks the cone, so the plume’s advance toward Mocho slows and may partially redistribute.
Stoneridge wells
• Hydro role: Located closer to the Pleasanton/Dublin urban corridor, tapping similar alluvial deposits.
• PFAS interaction:
o With PFAS detected, treatment allows continued pumping—but hydraulically, the wells still act as sinks, drawing PFAS bearing water along preferential pathways.
o Over time, this can capture part of the plume but also pull it farther from the original source area.
Chain of Lakes wellfield
• Hydro role: Large, strategically located wellfield near the Chain of Lakes recharge/storage area.
• PFAS interaction:
o High capacity pumping here strongly influences regional gradients, potentially capturing PFAS moving down valley.
o Think of it as a big drain—if PFAS is in the flow path, it will tend to be drawn toward these wells.
Other Pleasanton/retailer wells
• Any municipal well screened in PFAS impacted zones will:
o Distort the local plume, pulling it toward the well.
o Potentially connect shallow and deeper contamination if the well is long screened and vertical gradients exist.
3. Geologic “columns”: vertical and horizontal pathways
When you say “vertical and horizontal columns”, here is what that looks like in real geology:
Horizontal pathways (lateral “pipes”)
• Coarse sand and gravel lenses in alluvial fans.
• Buried paleo channels—old stream channels filled with permeable material.
• These form laterally continuous, high K zones that:
o Carry PFAS farther and faster horizontally.
o Connect distant parts of the basin more efficiently than surrounding finer sediments.
Vertical pathways (chimneys)
• Fractures and faults in consolidated sediments or near basin margins.
• Cross cutting coarse lenses that pierce through finer layers.
• Leaky confining beds where clays/silts are discontinuous or thin.
• These allow PFAS to:
o Leak downward from shallow contamination into deeper production zones.
o Bypass what would otherwise be semi confining layers.
Retarding but not protective layers
• Silt and clay beds:
o Slow groundwater flow and PFAS migration.
o Can cause perched or semi perched zones.
o But PFAS is persistent—so these layers delay, rather than permanently block, migration.
There are effectively vertical and horizontal “columns” in the Tri Valley aquifer system, and they absolutely matter for PFAS plume behavior.
4. How pumping reshapes the PFAS plume over time
Think of the plume as a smoke cloud and the wells as vacuum cleaners:
• Cones of depression:
o Each pumping well creates a zone of lower hydraulic head.
o PFAS bearing water flows toward that low point, bending the plume.
• Stretching and focusing:
o A line of wells (e.g., a wellfield) can stretch the plume along the flow direction and narrow it perpendicular to that line.
o Preferential pathways amplify this—plume fingers can extend along specific channels.
• Vertical capture:
o Deep wells can pull water from above, especially if the well is long screened.
o If PFAS is mainly shallow, pumping can drag it downward into deeper zones that were initially clean.
• Operational changes:
o Reducing pumping at a PFAS impacted wellfield (like Mocho) reduces its “vacuum effect,” so:
 The plume may slow in that direction.
 Flow may re route toward other active wells.
o Adding treatment does not change the hydraulics, but it changes the risk—you are still pulling PFAS toward the well, but now you’re removing it at the surface instead of sending it to taps.
Key ideas in that sketch:
• PFAS starts shallow, moves laterally along permeable layers.
• Where vertical pathways exist, it leaks downward.
• Pumping wells pull water from both shallow and deep zones, depending on screen length and gradients.
• Over time, the plume bends toward and stretches around the wellfield.
• Risk to specific wells?
• How fast might PFAS move vertically?
• Or how management actions (turning wells on/off, adding recharge) could be used to steer the plume?
1. Risk to specific wells in the Tri Valley context
Big picture: Any well that is (a) hydraulically downgradient of PFAS sources and (b) screened in connected aquifer zones is at risk. In the Tri Valley/Livermore–Pleasanton basin, that especially means:
• Mocho Wellfield (Pleasanton area)
o Risk drivers:
 Located along a major flow path (Arroyo Mocho corridor).
 Likely connected to both shallow and intermediate alluvial zones.
o Risk profile:
 If PFAS is present upgradient or shallow, Mocho’s pumping can pull it laterally and vertically into the screened intervals.
 Reduced pumping lowers immediate risk but doesn’t eliminate PFAS already in the system.
• Stoneridge wells
o Risk drivers:
 Urban setting, potential proximity to historical PFAS uses.
 Pumping from permeable alluvium with possible preferential pathways.
o Risk profile:
 PFAS already detected historically → confirms hydraulic connection to impacted zones.
 With treatment, water quality risk is mitigated, but subsurface plume interaction continues.
• Chain of Lakes wellfield
o Risk drivers:
 High capacity pumping strongly shapes regional gradients.
 Located along a key down valley flow path.
o Risk profile:
 If PFAS plumes migrate down valley, this field can capture them but also concentrate PFAS mass in the vicinity of the wells (handled by treatment).
• Other Pleasanton/retailer wells
o Risk drivers:
 Long screened intervals that may intersect multiple aquifer layers.
 Locations along the valley axis or near arrays.
o Risk profile:
 At risk if they sit in the hydraulic shadow of PFAS sources (i.e., downgradient and connected by permeable pathways).
Key takeaway: Risk is highest where preferential pathways + downgradient position + strong pumping all overlap.
2. How fast PFAS might move vertically
Vertical movement is controlled by hydraulic gradients + permeability + layering. In a basin like Tri Valley:
• Where clays/silts are thick and continuous:
o Vertical migration is slow years to decades to move through or around a confining layer.
o PFAS may accumulate above these layers in perched or semi perched zones.
• Where vertical pathways exist (fractures, coarse lenses, leaky spots):
o Vertical migration can be much faster—on the order of years rather than decades to move from shallow to intermediate depths.
o Strong downward gradients from pumping wells can accelerate this.
• Influence of well construction:
o Long screened wells can act like artificial vertical conduits, allowing water from different depths to mix inside the wellbore.
o That means PFAS from a shallower zone can effectively be short circuited into deeper parts of the aquifer.
So, there isn’t a single “speed”—you have:
• Slow vertical leakage through fine grained layers.
• Fast vertical jumps where preferential pathways or well construction provide shortcuts.
In a mixed alluvial system like Tri Valley, you can realistically expect meaningful vertical migration over a decade scale timeframe, especially near pumping centers.
3. How management actions can steer or contain the plume
You actually have a surprising amount of control over plume behavior through hydraulic and operational management:
A. Pumping strategies
• Reduce or shut down high risk wells.
o Lowers the local cone of depression → reduces the pull on PFAS toward that well.
o Can slow vertical migration into deeper zones.
• Shift pumping to treat wells in better positions
o If a treated wellfield (e.g., with PFAS IX treatment) is placed down gradient of the plume, you can:
 Hydraulically capture more of the PFAS mass.
 Protect other wells by drawing the plume toward the treated “sink.”
B. Well design and rehabilitation
• Shorten screens or isolate intervals
o New or retrofitted wells with shorter, depth targeted screens reduce vertical mixing.
o Cementing off shallow intervals in existing wells can limit vertical short circuiting.
• Add sentinel wells
o Shallow and intermediate monitoring wells between sources and production wells give early warning of plume movement.
o Data from these wells can guide when and where to adjust pumping.
C. Managed aquifer recharge and flow steering
• Recharge in clean upgradient areas
o Can flatten or reverse gradients locally, pushing PFAS away from sensitive wells.
• Avoid recharge near PFAS sources
o Extra recharge near a source can drive PFAS deeper or farther—so siting is critical.
D. Operational playbook (conceptual)
1. Map the plume (shallow + deep) and identify preferential pathways.
2. Rank wells by risk (downgradient position, screen depth, PFAS detections).
3. Reduce or modify pumping at the highest risk wells.
4. Increase pumping at treated wells positioned to capture the plume.
5. Refine well designs over time (shorter screens, interval isolation).
6. Continuously update the groundwater model with monitoring data to see how the plume responds.