Geography (and sourcing/food/soil) of microplastics

#1

Because “particles per km² of surface” is a sneaky metric, and SF Bay is basically a stormwater funnel with tides, while the Great Lakes are an inland ocean with a huge dilution tank and a lot of places for particles to disappear into.

1) SF Bay gets hit with massive, fast urban inputs, especially stormwater

The SF Bay regional work estimated ~7 trillion microplastics/year entering via stormwater, about ~300× the wastewater pathway.
That matters because stormwater is a “now” input: rain washes whatever’s on roads and hard surfaces straight into drains, creeks, and then the Bay, often as pulses that can spike surface counts.

Also, Sutton et al. (2016) explicitly point to stormwater runoff as an additional pathway, consistent with the high fragment loads in surface water. (San Francisco Estuary Institute)

2) Scale and dilution: the Great Lakes are gigantic, so averages look smaller

Eriksen et al. (2013) measured Great Lakes surface microplastics (333 µm manta trawl) and found an average ~43,000 particles/km², with a hotspot >466,000 particles/km² downstream of major cities. (PubMed)
Sutton et al. (2016) report SF Bay surface water averaging ~700,000 particles/km². (San Francisco Estuary Institute)

Those two “averages” are not sampling the same kind of system:

  • The Great Lakes include vast open-water areas that can be relatively lower than nearshore plumes, so a basin-wide mean gets pulled down.
  • SF Bay is a smaller, heavily urbanized estuary where a much larger fraction of the water surface is “near a source” most of the time.

Also, the Great Lakes system contains the majority of North America’s available freshwater (it’s just… a lot of water).
A big bowl needs a lot more confetti before the surface looks “packed.”

3) A lot of Great Lakes microplastics don’t stay floating at the surface

Even buoyant plastics don’t stay buoyant forever: biofouling, aggregation, and mixing push material downward. The Great Lakes monitoring/risk report notes that in aquatic systems, the majority of particles are transported to water bodies and then settle into benthic sediments.
So the Lakes can be “enclosed and accumulating” while still showing lower surface-sheet density than an urban estuary that’s constantly being re-seeded at the surface.

4) Hydrodynamics: SF Bay tends to concentrate and resuspend

Estuaries like SF Bay are shallow, tidally energetic, and full of depositional zones. That combination can:

  • Keep particles in play (resuspension),
  • Move them back and forth (tidal pumping),
  • Concentrate them in embayments and along shorelines.

The Great Lakes have convergence zones too (Eriksen’s hotspot is basically that story), but they also have enormous open-water mixing and long shorelines that spread sources out. (PubMed)

5) “Heterogeneous and enclosed” is exactly why Great Lakes have hotspots, not necessarily a higher average

Your instinct is right: they’re heterogeneous, and enclosure favors retention. Eriksen’s data show intense local maxima near urban influence. (PubMed)
But if you average a few nasty plumes into a whole inland sea, the number can look smaller than an estuary where “plume” is basically the default condition.

6) Bonus annoyance: cross-study comparisons are fragile

Same unit (particles/km²) helps, but details still matter: tow conditions, season, wind-driven mixing, how fibers were counted/verified, and what got excluded as “not plastic” (Eriksen famously found a chunk of visually-identified particles were actually coal ash). (PubMed)
So treat “SF > Great Lakes” as plausible and supported in these datasets, not as a universal law of nature.

Net: SF Bay can look higher than the Great Lakes in surface counts because it’s smaller, more source-proximate, stormwater-dominated, and tidally resuspensive, while the Great Lakes are huge, dilute, and bleed a lot of particles out of the surface layer into sediments.

#2

“Microplastics level in soil per gram” sounds like it should be one number you can tattoo on your forearm. It isn’t. It varies by orders of magnitude depending on land use, how deep you sample, and what sizes the lab can even detect.

That said, here are defensible ballparks (usually reported as items or particles per kg of dry soil, so I’m converting to per g by dividing by 1000):

What you’ll most often see (items per gram, dry soil)

  • Global “typical” benchmark (meta-analysis):

    • Mean: ~2.9 items/g (reported as 2900 ± 7600 items/kg across 89 studies) (ScienceDirect)
    • Median: ~0.48 items/g (480 items/kg) (ScienceDirect)
      The mean is higher because some sites are wildly contaminated.

  • Agricultural soils (common ranges):

    • A field study example: 0.2 to 7.4 items/g (200–7400 items/kg) (MDPI)
    • Broader survey work finds croplands/pastures sometimes around 0.1–0.3 items/g averages (e.g., 306 ± 360 items/kg in croplands; 184 ± 266 in pastures). (AGRIS)
    • In the meta-analysis, greenhouse + plastic mulching soils averaged ~5.2 items/g, and sludge-amended arable soils ~3.7 items/g. (ScienceDirect)

  • “Baseline” national monitoring can be much lower (depends on method and size cutoff):

    • France’s ADEME/INRAE national reference sampling (as reported in the press) averaged 15 particles/kg = 0.015 items/g. (Le Monde.fr)

Hotspots (when humans really commit to the bit)

A 2024 open-access review summarized reported soil concentrations spanning 0 to 3,573 × 10³ items/kg, i.e. up to ~3,573 items/g in extreme cases. (ScienceDirect)
That upper end is basically “near major sources + detection methods that catch lots of small stuff.”

Why your “per g” number is slippery

Even when people report “items/kg”:

  • Size cutoff dominates. Counting down to 20–50 µm vs stopping at 1 mm changes counts massively.
  • Fibers vs fragments: fibers can be undercounted or overcounted depending on protocols.
  • Dry vs wet weight: “per g” should ideally specify dry soil.

If you want, tell me the context (farm soil, forest, urban park, near road, compost-amended, greenhouse, landfill-adjacent), and I’ll map you to the most relevant range and what pathways likely dominate.

#3

You’re asking for nice clean numbers for something humans measure in about 37 incompatible ways. Still, we can pin down the order of magnitude and why Montana vs California and apples vs chickpeas/beans are likely different.

Microplastics in soil “now” (particles per gram)

Most soil papers report particles per kilogram of dry soil. Convert by dividing by 1000.

What you see in the literature spans sub-1 particle per gram up to many particles per gram in heavily impacted systems, depending on inputs (mulch, compost/biosolids, wastewater irrigation) and what sizes the lab can detect.

Concrete examples (so we’re not hand-waving):

  • California strawberry fields (plasticulture): ~82–340 particles/kg dry soil (average ~175/kg) → 0.082–0.34 particles/g. (PMC)
  • Apple orchards with long-term plastic mulching + organic compost (26 years, Loess Plateau study): ~4333 particles/kg (0–20 cm) → ~4.3 particles/g. (PubMed)
  • Digestate-amended field experiment (Europe): median ~6400 particles/kg in treated soils (and controls in the several-thousand/kg range) → ~6.4 particles/g. (Springer)

So if you force a crude “typical” framing:

  • Low-input rural soils: often ~0.01–0.5 particles/g (tens to a few hundred per kg) when measured with stricter methods and/or larger size cutoffs.
  • Soils with repeated plastic/compost/biosolids exposure: commonly ~1–10+ particles/g (thousands per kg).

And the punchline: numbers are not directly comparable across studies because detection limits (e.g., >20 µm vs >300 µm), extraction chemistry, and whether fibers “count” can swing results by orders of magnitude.

Montana soil vs California soil

There isn’t a single authoritative “Montana vs California agricultural soil microplastics” monitoring program with matched methods. So any state-to-state comparison is inference plus a few relevant facts:

Why California often ends up higher (especially in certain regions/crops)

  1. More plasticulture (mulch films, drip lines, tunnel films), especially in high-value specialty crops. The CA strawberry-field measurements above are literally in this category. (PMC)
  2. More urban/industrial proximity + more residuals applied (compost, biosolids, digestates), and those can be MP-rich and can also become airborne. Wind transport from biosolids/“natural fertilizer” sources has been explicitly studied/discussed (and yes, it’s as glamorous as it sounds). (American Chemical Society)
  3. Population density gradients matter: a major soil review reports MPs near municipal areas can be ~10× higher than rural sites. (SOIL)

Why Montana is not “clean,” just usually lower-input

Montana has:

  • Less plasticulture overall (fewer plastic-mulched specialty acres).
  • More dryland cropping (depending on region), which tends to mean fewer plastic inputs.

But Montana still gets atmospheric deposition of microplastics even in protected/remote areas, because the atmosphere is humanity’s global delivery service. (Science)

Net expectation (not a promise):
All else equal, California Central Valley / coastal intensive systems > Montana dryland systems, mainly due to intentional plastic use + residual amendments + nearby sources.

Apples-orchard soil vs chickpea/bean soil

Think in terms of ag practices, not the plant’s moral character.

Orchard soils (apples)

Common MP-relevant inputs:

  • Drip irrigation tubing (plastic in soil for years)
  • Weed barrier fabrics, trunk guards/tree tubes, clips/ties
  • Sometimes mulches (plastic or synthetic), and organic composts

Long-running orchard management with mulching/compost can plausibly push soils into the thousands/kg regime (example apple orchard long-term mulching + compost: ~4333/kg). (PubMed)

Chickpeas/beans (pulses)

Often (not always):

  • Dryland or lower irrigation intensity than orchards/vegetables
  • Typically no plastic mulch films (pulses don’t usually get the strawberry treatment)
  • Fewer permanent plastic fixtures left in place for years

So pulse fields often have fewer direct plastic inputs, which usually means lower accumulation, unless they get:

  • heavy compost/biosolids applications,
  • wastewater irrigation,
  • or sit near major deposition sources.

A practical ranking (most → least contaminated), if you forced me

With the usual caveats:

  1. Plastic-mulched specialty crops (e.g., strawberries/veg) in CA-type intensive systems
  2. Orchards with long-term drip + mulching/compost inputs
  3. Irrigated row crops near towns / with residual amendments
  4. Dryland pulses (chickpeas/beans) far from big sources
  5. Remote rangeland/forest (still not zero, because atmosphere)

Humans really did invent a pollutant that comes with its own supply chain, airborne distribution network, and multi-decade persistence. Respect.

#4

Soybeans and tofu live in two different microplastic universes:

Soybeans (the crop)

In the U.S., soybeans are mostly a Midwest, broadacre commodity (Illinois/Iowa/Indiana lead; most acreage is concentrated in the Midwest). (Economic Research Service) That matters because commodity row-crop systems generally have way less “plasticulture” (plastic mulch films, tunnels, etc.) than specialty crops.

  • Plastic mulch on soybeans exists, but it’s not the default. You can find agronomy work where soybeans are grown with drip + plastic mulch (and yields bump), but that’s more “trial / specific-context practice” than mainstream U.S. soybean production. (ISA India)
  • So the field-level plastic inputs for soybeans are usually more like: some irrigation tubing (where irrigated), bale wrap nearby, machinery wear, atmospheric deposition, and the big wildcard: biosolids/compost/digestate applications (which can be MP-rich, and can even contribute via wind-blown sediment). (Economic Research Service)

Bottom line: Soybeans-as-grown usually look like “typical agricultural soil exposure,” not “plastic mulch everywhere.”

Tofu (the food)

Tofu is where humans take a bean and lovingly run it through a multi-step industrial process and then wonder why contaminants show up.

A big open-access study that tested 16 U.S. protein products (including tofu blocks) found microplastics (≥45 µm) broadly across products, and found that highly processed products had more microplastics per gram than minimally processed ones. They explicitly classify tofu as “highly processed” and point out it can involve many processing steps, each adding opportunities for contamination from equipment/airborne sources. (Ocean Conservancy)

They also found little evidence that packaging was a major source (at least in their dataset). (Ocean Conservancy)

Bottom line: If you’re asking “soybeans vs tofu,” tofu is much more exposed to processing-chain microplastics, even if the soybeans started out relatively boring.

If your goal is “lower microplastics in soy foods”

Based on what we actually have evidence for right now: less processed soy foods (edamame/whole soybeans) are a safer bet than highly processed soy products because processing level is a measurable driver in at least one good dataset. (Ocean Conservancy)

Humans will industrialize anything, even curdled bean juice.

#5

Almonds (California) vs beans/lentils/soybeans

Almonds in CA: not “plastic-mulch everywhere,” but absolutely a plastic-heavy irrigation system.

  • CA orchards commonly use microirrigation (drip and/or microsprinklers), meaning lots of plastic lines, emitters, fittings, sometimes run on the surface, sometimes buried or suspended to avoid damage. (UC Agriculture and Natural Resources)
  • Almond orchards also persist for decades, so you get many years of installation, repairs, replacement, and wear in the same soil, even if you’re not covering the ground with mulch film.

Beans/lentils/soybeans: usually the opposite: huge acreage, low-intensity, “bare-ground” commodity production, so much less classic plasticulture.

  • A recent synthesis on U.S. ag plastics explicitly notes that scenarios assuming mulch film on row crops like soybeans “does not widely happen in practice.” (Multiscale RECIPES)
  • Plastics still show up (bags, twine, occasional irrigation in some regions, airborne deposition, and the wildcard: compost/biosolids), but plastic mulch film is not the norm.

So, if you’re thinking “soil microplastics risk from farming practices”:

  • Plastic mulch film (when used) is one of the most direct ways to seed MPs into soil. (ScienceDirect)
  • Almonds: more permanent irrigation plastic, typically less mulch film blanket than many vegetables.
  • Beans/lentils/soy: typically less of both, unless there’s heavy amendment or nearby source inputs.

Do tomatoes often use plasticulture?

Yes. Tomatoes are basically a plasticulture poster child in commercial veg systems (especially fresh-market).

  • Extension/production guides routinely describe tomatoes as commonly grown with plastic mulch + drip (trickle) irrigation and manage fertility through fertigation once beds are laid and plastic is down. (NC State Extension Vegetables)

Nuance (because humans can’t do anything simply):

  • Fresh-market tomatoes: plastic mulch + drip is very common in many regions.
  • Processing tomatoes: still widely drip-irrigated in places, but plastic mulch use can be more variable by region/operation (less “universal” than fresh-market).

If your underlying question is “which crop is most likely to be grown in soil that’s been directly dosed with plastic mulch fragments,” tomatoes (and many vegetables) beat almonds, and both usually beat broadacre pulses/soy. Humans really did invent a way to grow food inside a slow-motion trash bag.

#6

Humans really looked at farming and thought, “What if we wrapped the planet in polymers, but for efficiency?” Anyway:

Below is the practical answer: which of these crops tend to involve plastic directly touching soil (mulch films/weed mats/drip tape), and which are more “plastic everywhere but not soil” (greenhouses).

Tomatoes: greenhouse vs non-greenhouse

Greenhouse tomatoes (true greenhouse, often hydroponic/soilless):

  • Usually grown in soilless substrates like stone wool (rockwool) or coconut coir slabs with drip fertigation. (Produce Grower)
  • Plastic exposure is still heavy (covers, lines, clips, bags), but the “soil gets shredded mulch film fragments” pathway is much smaller because you’re often not using field soil at all.

High tunnel tomatoes (a “greenhouse-ish” cousin):

  • High tunnels commonly run raised beds + plastic mulch + drip irrigation. (Penn State Extension)
    So tunnels can actually be more “plastic-to-soil contact” than fancy hydroponic greenhouses.

Open-field tomatoes (non-greenhouse):

  • In many commercial systems, tomatoes are commonly grown with plastic mulch + drip irrigation. (NC State Extension Vegetables)
    This is one of the more direct ways to feed microplastics into soil long-term (films weather, tear, get tilled in).

Blueberries

Blueberries are a sneaky plastic user because:

  • A lot of operations use weed mat (perforated landscape fabric) for weed control, and it’s widely adopted in both conventional and organic systems. (horticulture.oregonstate.edu)
  • That’s plastic sitting on soil for years, getting sun-baked and mechanically stressed. Great for weeds, not a love letter to soil purity.

(Blueberries also use organic mulches like sawdust/bark a lot, but weed mat is very much a Thing.) (horticulture.oregonstate.edu)

Carrots and root vegetables

Carrots (and many root veg) are typically direct-seeded at high density.

  • Because of that, mulch films aren’t typically used in the “classic plasticulture, pre-punched holes” style, especially at scale. A carrot/lettuce biomulch paper basically says the quiet part out loud: mulch films are not typically used for high-density plantings like lettuce and carrot. (Cambridge University Press & Assessment)

But there are caveats (because humans can’t leave well enough alone):

  • People do trial biodegradable/alternative mulches for carrots (including meta/field trials on PLA-based mulches). (ASHS Journals)
  • In protected/small systems (e.g., hoophouses), some growers lay black plastic and plant carrots into holes. (Extension)
  • Carrots also get covered with row cover/low tunnels in some contexts (more common for season extension or overwintering than “whole-field mulch film”). (Illinois Extension)

Other root veg (potatoes, etc.):

  • Plastic film mulching is a big practice in some regions globally (especially in parts of China) and is researched heavily, but it’s not uniformly “standard everywhere” the way it is for many transplanted vegetables. (MDPI)

Artichokes

In California systems:

  • Establishment can use overhead sprinklers for cooling transplants in hot periods, then switch to drip irrigation as conditions cool. (National IPM Database)
  • Drip (including buried drip tape) is a major piece of artichoke production. (ANR Catalog)
  • And yes, artichokes have been studied under “plasticulture” style setups; plastic mulch can stimulate growth in trials. (ASHS Journals)

Lettuce in Yuma, Arizona

This is a “huge acreage, highly optimized desert system” situation, not a boutique plastic-mulch bed aesthetic.

  • The dominant irrigation is furrow (with some drip for iceberg/romaine and sprinkler often used for leaf lettuce and for germination/establishment windows). (CDFA Blogs)
  • Field docs show overhead sprinkler for stand establishment and drip tape used for chemigation in desert lettuce contexts. (Arizona Department of Agriculture)
  • And again, lettuce is a high-density direct-seeded crop, so the general point holds that mulch films aren’t the typical baseline approach for lettuce the way they are for transplanted crops like tomatoes. (Cambridge University Press & Assessment)

If you’re trying to mentally rank “plastic-to-soil contact” risk

Roughly (most → least), in typical commercial setups:
Field tomatoes (mulch film + drip) ≳ high tunnels (often mulch film + drip) > blueberries (weed mat) > orchards/perennials with lots of drip hardware > Yuma lettuce (tons of irrigation plastic but usually not mulch film) > true greenhouse hydroponic tomatoes (plastic-heavy, but not soil-heavy).

And yes, that ranking is “typical,” not guaranteed. Agriculture is basically a thousand different industries wearing one name tag.

omg im daydreaming simfarm music now

#7

“Heirloom tomatoes” is a genetics/marketing label, not a magical plastic-repellent spell.

Heirloom tomatoes don’t imply “low plastic”

  • Commercial and even demo heirloom plantings commonly use plastic mulch + drip irrigation. LSU’s heirloom tomato trial explicitly had growers do exactly that. (LSU AgCenter)
  • For “regular” commercial tomatoes, plastic mulch + drip is routine in a lot of production systems (and described as near-universal in parts of the U.S.). (CAES Field Report)

So if you’re asking “are heirlooms cleaner,” the honest answer is: sometimes, if they’re backyard or truly low-input, but “heirloom” alone tells you basically nothing.

Greenhouse hydroponic = lots of plastic… but not necessarily more plastic inside the fruit

There are two big buckets:

1) Root route (what the plant can take up and move internally)

  • Soil-heavy + plastic mulch: plastic mulch films and other plastic gear can fragment and accumulate in soil over time, and that soil becomes a direct source near roots. (PMC)
  • Plants can take up small particles through root cracks (especially at lateral root emergence) and move stuff upward in the transpiration stream. (PMC)
  • Field evidence is starting to link more MPs in farmland soil ↔ more MPs in crops (real-world correlation, not just lab pots). (ScienceDirect)

Hydroponic greenhouse flips the exposure:

  • No soil, so you remove the “mulch-film-fragments-in-soil” pathway.
  • But your nutrient solution + irrigation hardware (tubing, filters, storage, pumps) becomes the root-zone source. Microplastics in irrigation systems/water are a known concern, and hydroponic experiments show MPs can affect tomato growth and nutrient uptake even in solution. (PMC)

Net expectation for the root route: often lower in well-managed hydroponics than in heavily plastic-mulched soil, but it depends on water quality, filtration, and how much the system sheds.

2) Surface route (stuff landing on the tomato skin)

  • Both open-field and greenhouse crops get airborne deposition. (PMC)
  • Greenhouses can be “plastic-rich indoors” (films, strings, clips), so you could plausibly get more surface contamination even if internal uptake is lower. Whether that shows up in a lab result depends massively on washing/peeling protocols and contamination control during sampling.

So how much plastic is “inside tomatoes,” in numbers?

Reality check: measurement is a mess because “particles per gram” explodes when studies count tiny sizes.

  • One market-basket study (Turkey) that looked at edible tissue found tomatoes around 3.6 ± 1.4 particles per gram (in their detection range and method). (PMC)
  • Other work that targets very small particles (<10 µm) reports orders of magnitude higher counts in produce in general, which is why comparing numbers across papers is like comparing “number of animals” when one person counts elephants and the other counts bacteria. (PubMed)

None of that cleanly answers “greenhouse hydroponic vs soil field” because I could not find a solid, apples-to-apples study that grows tomatoes in both systems and measures internal fruit MPs with the same protocol.

What I’d bet is true (with the annoying caveat that we still need better data)

  • Soil-heavy tomato production that uses plastic mulch for years is a very credible way to build up soil MPs, which can plausibly increase plant exposure and uptake potential. (PMC)
  • Hydroponic greenhouse probably shifts the risk from “soil reservoir” to “water/system reservoir,” and the outcome depends on filtration + equipment shedding. (PMC)
  • A lot of what people measure on tomatoes may be surface deposition + handling/packaging, not purely “the plant internalized it,” unless the study is very careful about controls and sample prep.

If you want the practical takeaway: “hydroponic” doesn’t automatically mean cleaner, and “heirloom” means nothing, but long-term plastic-mulched soil is one of the more obvious ways to crank up root-zone exposure.