EXHIBIT 23.1

                        REPLACEMENT LINE OF CREDIT NOTE

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Borrower Name
EMCON
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Borrower Address                          Office              Loan Number
  400 South El Camino Real, Suite 1200    70061
  San Mateo, California  94402-1708
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                                          Maturity Date       Amount
                                          April 30, 1999      $5,000,000.00
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$5,000,000.00                                     Effective as of March 19, 1999

FOR VALUE  RECEIVED,  on April  30,  1999  ("Maturity  Date"),  the  undersigned
("Borrower")  promises  to pay to the order of UNION  BANK OF  CALIFORNIA,  N.A.
("Bank"),  as  indicated  below,  the  principal  sum of FIVE MILLION AND NO/100
DOLLARS  ($5,000,000.00),  or so much  thereof as is  disbursed,  together  with
interest on the balance of such principal from time to time outstanding,  at the
per annum rate or rates and at the times set forth  below.  At any time prior to
the maturity of this Note,  subject to the provisions of paragraph 4, below,  of
this Note,  Borrower may borrow,  repay and reborrow hereon so long as the total
outstanding  at any one time does not exceed the principal  amount of this Note.
All  computations  of  interest  under this Note shall be made on the basis of a
year of 360 days, for actual days elapsed.

1. INTEREST PAYMENTS.  Borrower shall pay interest on the 19TH day of each month
commencing the first such date to occur after the first advance under this Note.
Should  interest not be paid when due, it shall become part of the principal and
bear  interest as herein  provided.  On the Maturity  Date,  all  principal  and
interest then unpaid shall be due and payable.

         (a) BASE  INTEREST  RATE. At Borrower's  options,  amounts  outstanding
         hereunder in increments of at least  $100,000.00 shall bear interest at
         a rate,  based on an index  selected by  Borrower,  which is 1-3/4% per
         annum in excess of Bank's LIBOR Rate for the Interest  Period  selected
         by Borrower, in each case acceptable to Bank.

         No Base  Interest  Rate may be changed,  altered or otherwise  modified
         until the expiration of the Interest Period  selected by Borrower.  The
         exercise of interest  rate options by Borrower  shall be as recorded in
         Bank's  records,  which records  shall be prima facile  evidence of the
         amount  borrowed  under either  interest  option and the interest rate;
         provided,  however,  that failure of Bank to make any such  notation in
         its records shall not discharge  Borrower from its obligations to repay
         in full with  interest  all  amounts  borrowed.  In no event  shall any
         Interest Period extend beyond the maturity date of this Note.

         To exercise  this option,  Borrower may, from time to time with respect
         to principal outstanding on which a Base Interest Rate is not accruing,
         and on the expiration of any Interest  period with respect to principal
         outstanding on which a Base Interest Rate has been accruing,  select an
         index  offered by Bank for a Base  Interest  Rate Loan and an  Interest
         Period by telephoning an authorized  lending officer of Bank located at
         the banking office identified below prior to 10:00 a.m.,  Pacific time,
         on any Business Day and  advising  that officer of the selected  Index,
         the  Interest   Period  and  the   Origination   Date  selected  (which
         Origination  Date,  for a Base  Interest  Rate Loan  based on the LIBOR
         Rate,  shall follow the date of such  selection by no more than two (2)
         Business Days).


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         Bank will mail a written  confirmation of the terms of the selection to
         Borrower  promptly  after the  election  is made.  Failure to send such
         confirmation  shall not affect Bank's rights to collect interest at the
         rate selected. If, on the date of the selection,  the index selected is
         unavailable for any reason,  the selection shall be void. Bank reserves
         the   right  to  fund  the   principal   from  any   source   of  funds
         notwithstanding any Base Interest Rate selected by Borrower.

         (b) VARIABLE INTEREST RATE. All principal  outstanding  hereunder which
         is not bearing  interest at a Base Interest Rate shall bear interest at
         a rate per annum equal to the Reference Rate,  which rate shall vary as
         and when the Reference Rate changes.

         Borrower  shall pay all amounts due under this Note in lawful  money of
         the United  States at Bank's  Northern  California  Commercial  Banking
         Office, or such other office as may be designated by Bank, from time to
         time.

         2. LATE  PAYMENTS.  If any  payment  required by the terms of this Note
         shall remain  unpaid ten days after same is due, at the option of Bank,
         Borrower shall pay a fee of $100 to Bank.

         3. INTEREST RATE  FOLLOWING  DEFAULT.  In the event of default,  at the
         option of Bank, and, to the extent permitted by law,  interest shall be
         payable  on the  outstanding  principal  under this Note at a per annum
         rate  equal to five  percent  (5%) in  excess  of the  Reference  Rate,
         calculated  from the date of default  until all amounts  payable  under
         this Note are paid in full.

         4.       PREPAYMENT.

                  (a) Amounts  outstanding under this Note bearing interest at a
                  rate based on the Reference Rate may be prepaid in whole or in
                  part at any time,  without  penalty or premium.  Borrower  may
                  prepay amounts outstanding under this Note bearing interest at
                  a Base Interest Rate in whole or in part provided Borrower has
                  given Bank not less than five (5) Business  Days prior written
                  notice of  Borrower's  intention to make such  prepayment  and
                  pays  to  Bank  the  liquidated   damages  due  as  a  result.
                  Liquidated  Damages shall also be paid, if Bank, for any other
                  reason, including acceleration or foreclosure, receives all or
                  any portion of principal  bearing  interest at a Base Interest
                  Rate prior to its scheduled payment date.  Liquidated  Damages
                  shall be an amount  equal to the present  value of the product
                  of: (i) the  difference  (but not less than zero)  between (a)
                  the Base  Interest Rate  applicable  to the  principal  amount
                  which is being  prepaid,  and (b) the return  which Bank could
                  obtain if it used the amount of such  prepayment  of principal
                  to purchase at bid price regularly quoted securities issued by
                  the  United   States  having  a  maturity  date  most  closely
                  coinciding  with the relevant Base Rate Maturity Date and such
                  securities  were  held by Bank  until the  relevant  Base Rate
                  Maturity Date ("Yield Rate");  (ii) a fraction,  the numerator
                  of which is the number of days in the period  between the date
                  of prepayment and the relevant Base Rate Maturity Date and the
                  denominator  of which is 360;  and  (iii)  the  amount  of the
                  principal  so  prepaid  (except  in the event  that  principal
                  payments are  required  and have been made as scheduled  under
                  the terms of the Base Interest Rate Loan being  prepaid,  then
                  an amount equal to the lesser of (A) the amount prepaid or (B)
                  50% of the sum of (1) the amount prepaid and (2) the amount of
                  principal  scheduled under the terms of the Base Interest Rate
                  Loan being prepaid to be outstanding at the relevant Base Rate
                  Maturity Date). Present value under this Note is determined by
                  discounting the above product to present value using the Yield
                  Rate as the annual discount factor.


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                  (b) In no event shall Bank be obligated to make any payment or
                  refund to  Borrower,  nor shall  Borrower  be  entitled to any
                  setoff or other claim  against  Bank,  should the return which
                  Bank could obtain  under this  prepayment  formula  exceed the
                  interest that Bank would have received if not  prepayment  had
                  occurred.  All  prepayments  shall include  payment of accrued
                  interest  on the  principal  amount  so  prepaid  and shall be
                  applied  to  payment  of  interest   before   application   to
                  principal.  A  determination  by Bank as to the prepayment fee
                  amount, if any, shall be conclusive.

                  (c) Bank  shall  provide  Borrower a  statement  of the amount
                  payable on account of prepayment.  Borrower  acknowledges that
                  (1)  Bank   establishes   a  Base   Interest   Rate  upon  the
                  understanding that it apply to the Base Interest Rate Loan for
                  the entire Interest Period, and (ii) any prepayment may result
                  in Bank incurring  additional costs,  expenses or liabilities;
                  and  Borrower  agrees to pay  these  liquidated  damages  as a
                  reasonable estimate of the costs,  expenses and liabilities of
                  Bank associated with such prepayment.

         5. DEFAULT AND ACCELERATION OF TIME FOR PAYMENT. Default shall include,
         but  not be  limited  to,  any of the  following:  (a) the  failure  of
         Borrower to make any payment required under this Note when due; (b) any
         breach,  misrepresentation or other default by Borrower, any guarantor,
         co-maker,  endorser,  or any  person  or  entity  other  than  Borrower
         providing   security  for  this  Note  (hereinafter   individually  and
         collectively referred to as the "Obligor") under any security, guaranty
         or other agreement between Bank and any Obligor;  (c) the insolvency of
         any  Obligor  or the  failure  of any  Obligor  generally  to pay  such
         Obligor's  debts as such debts become due; (d) the  commencement  as to
         any Obligor of any voluntary or involuntary  proceeding  under any laws
         relating to bankruptcy, insolvency,  reorganization,  arrangement, debt
         adjustment or debtor relief;  (e) the assignment by any Obligor for the
         benefit  of  such  Obligor's   creditors;   (f)  the  appointment,   or
         commencement  of any  proceeding  for the  appointment  of a  receiver,
         trustee,  custodian or similar official for all or substantially all of
         any Obligor's property;  (g) the commencement of any proceeding for the
         dissolution  or  liquidation  of any Obligor;  (h) the  termination  of
         existence or death of any Obligor;  (i) the  revocation of any guaranty
         or subordination  agreement given in connection with this Note; (j) the
         failure of any Obligor to comply with any order, judgment,  injunction,
         decree, writ or demand of any court or other public authority;  (k) the
         filing  or  recording  against  any  Obligor,  or the  property  of any
         Obligor,  of any notice of levy,  notice to  withhold,  or other  legal
         process for taxes  other than  property  taxes;  (l) the default by any
         Obligor  personally liable for amounts owed hereunder on any obligation
         concerning  the  borrowing  of  money;  (m) the  issuance  against  any
         Obligor,  or the  property of any Obligor,  of any writ of  attachment,
         execution,  or other  judicial  lien; or (n) the  deterioration  of the
         financial  condition  of any  Obligor  which  results  in Bank  deeming
         itself,  in good  faith,  insecure.  Upon  the  occurrence  of any such
         default, Bank, in its discretion,  may cease to advance funds hereunder
         and may declare all  obligations  under this Note  immediately  due and
         payable;  however,  upon the occurrence of an event of default under d,
         e, f, or g, all  principal  and  interest  shall  automatically  become
         immediately due and payable.

         6. ADDITIONAL  AGREEMENTS OF BORROWER.  If any amounts owing under this
         note are not paid  when  due,  borrower  promises  to pay all costs and
         expenses, including reasonable attorneys' fees, incurred by bank in the
         collection or  enforcement  of this note.  Borrower and any endorser of
         this Note, for the maximum period of time and the full extent permitted
         by law, (a) waive diligence, presentment, demand, notice of nonpayment,
         protest,  notice of protest,  and notice of every  kind;  (b) waive the
         right to assert the defense of any statute of  limitations  to any debt
         or obligation hereunder;  and (c) consent to renewals and extensions of
         time for the payment of any amount due under this Note. If this Note is
         signed by more than one party, the term "Borrower"


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         includes  each  of the  undersigned  and  any  successors  in  interest
         thereof; all of whose liability shall be joint and several. Any married
         person who signs this Note agrees that  recourse may be had against the
         separate  property of that person for any  obligations  hereunder.  The
         receipt of any check or other item of payment by Bank,  at its  option,
         shall not be  considered a payment on account until such check or other
         item of payment is honored  when  presented  for  payment at the drawee
         bank.  Bank may delay the  credit of such  payment  based  upon  Bank's
         schedule  of funds  availability,  and  interest  under this Note shall
         accrue  until the funds are deemed  collected.  In any  action  brought
         under or arising out of this Note, Borrower and any Obligor,  including
         their successors and assigns, hereby consent to the jurisdiction of any
         competent  court  within the State of  California,  as  provided in any
         alternative dispute resolution  agreement executed between Borrower and
         Bank, and consent to service of process by any means authorized by said
         state's law. The term "Bank" includes,  without limitation,  any holder
         of this  Note.  This Note shall be  construed  in  accordance  with and
         governed  by the laws of the  State of  California.  This  Note  hereby
         incorporates any alternative dispute resolution  agreement  previously,
         concurrently or hereafter executed between Borrower and Bank.

         7.  DEFINITIONS.  As used herein,  the  following  terms shall have the
         meanings  respectively  set forth below.  "Base  Interest Rate" means a
         rate of interest  based on the LIBOR Rate.  "Base  Interest  Rate Loan"
         means amounts  outstanding under this Note that bear interest at a Base
         Interest  Rate.  "Base Rate  Maturity  Date"  means the last day of the
         Interest  Period with  respect to  principal  outstanding  under a Base
         Interest  Rate Loan.  "Business  Day" means a day on which Bank is open
         for business for the funding of corporate  loans,  and, with respect to
         the rate of interest based on the LIBOR Rate, on which dealings in U.S.
         dollar deposits outside of the United States may be carried on by Bank.
         "Interest  Period"  means with respect to funds  bearing  interest at a
         rate based on the LIBOR Rate, any calendar period of one, two, or three
         months.  In determining an Interest Period, a month means a period that
         starts on one  Business Day in a month and ends on and includes the day
         preceding the numerically  corresponding day in the next month. For any
         month in which there is no such numerically  corresponding day, then as
         to that  month,  such day shall be deemed to the last  calendar  day of
         such  month.  Any  Interest  Period  which  would  otherwise  end  on a
         non-Business  Day shall end on the next succeeding  Business Day unless
         that is the first day of a month,  in which event such Interest  Period
         shall end on the next preceding  Business Day. "LIBOR Rate" means a per
         annum rate of Interest  (rounded upward,  if necessary,  to the nearest
         1/100 of 1%) at which dollar deposits,  in immediately  available funds
         and in lawful  money of the  United  States  would be  offered to Bank,
         outside of the United States,  for a term  coinciding with the Interest
         Period  selected by Borrower  and for an amount  equal to the amount of
         principal  covered by Borrower's  interest rate selection,  plus Bank's
         costs,   including   the  cost,   if  any,  of  reserve   requirements.
         "Origination  Date"  means  the  first  day  of  the  Interest  Period.
         "Reference  Rate" means the rate announced by Bank from time to time at
         its corporate  headquarters as its "Reference Rate." The Reference Rate
         is an index  rate  determined  by Bank  from time to time as a means of
         pricing  certain  extensions of credit and is neither  directly tied to
         any external rate of interest or index nor  necessarily the lowest rate
         of interest charged by Bank at any given time.

         This  Note is the  Replacement  Line of  Credit  Note  defined  in that
         certain Extension and Modification Agreement dated as of March 19, 1999
         between  Bank  and  Borrower  (as  amended,   supplemented,   extended,
         restated, or renewed from time to time,  "Agreement"),  and is governed
         by the terms thereof,  and supersedes and replaces that certain Line of
         Credit  Note dated  January  27,  1999,  as amended  from time to time,
         executed by Borrower in favor of Bank (the "Previous  Note"). As of the
         effective date of the  Agreement,  all unpaid  principal,  interest and
         other amounts accrued and outstanding under the Previous Note shall for
         all purposes be and constitute unpaid


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         amounts outstanding under, and evidenced by this Note. Each capitalized
         term not  otherwise  defined in this Note shall  have the  meaning  set
         forth in the Agreement.

         EMCON, a California corporation

         By:  \s\  Eugene M. Herson                  
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         Title:  President and CEO                   

         By:  \s\ R. Michael Momboisse               
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         Title:  CFO and VP Legal                    



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