(Concurring in part, dissenting in part.) I agree with the trial court’s thoughtful opinion concluding that neither ch. 147, Stats., nor the Federal Water Pollution Control Act provides citizens with an absolute right of intervention in this kind of case. I would reverse and remand, however, for the trial court’s further consideration of the timeliness of the appellant’s motion to intervene under the mandatory provisions of sec. 803.09(1), Stats., because its analysis of the factors relevant to a determination under that section is incomplete.
I disagree with the majority’s statement (majority opinion, p. 188) that the record is silent as to the factors the trial court is required to consider in making its discretionary determination whether a motion to intervene is timely. Among the most important of those factors is the purpose of the attempted intervention. Natural Resources Defense Council v. Costle, 561 F.2d 904, 907 (D.C. Cir. 1977); Hodgson v. United Mine Workers of America, 473 F.2d 118, 129 (D.C. Cir. 1972). It is apparent from the record before us, and it is the express position of the Decade on this appeal, that its purpose in seeking intervention is to participate in the parties’ ongoing negotiations to change the terms of the stipulated judgment, and not to relitigate issues previously deter*193mined in this action. It is seeking, in effect, to implement the judgment rather than to defeat it.1
I also disagree with the majority’s assertion (majority opinion, p. 189) that the Decade has offered “no explanation” why it seeks intervention at this time. Its reason for doing so is its fear that the parties will negotiate all meaning out of the pollution-abatement timetable set forth in the stipulated judgment, resulting in additional pollution of Lake Michigan. This explanation is articulated in its motion to intervene2 and in its briefs on appeal. The basis for its fear is readily apparent from the record before us.
Attached to the motion to intervene is a series of correspondence between counsel for the Decade and counsel for the DNR commencing in June 1979, more than a year before the motion to intervene was made. This correspondence is expressly incorporated by reference into the Decade counsel’s affidavit in support of the motion. It reveals repeated and largely unsuccessful attempts by *194the Decade, which had been informed that the parties were negotiating amendments to the judgment, to discover the terms of the proposed changes so that it could determine whether to attempt to intervene. The Decade’s letters express its concern in clear terms that any changes in the stipulated judgment could adversely affect its members’ interests.3
The correspondence also contains a letter dated January 4, 1980, from counsel for the DNR to counsel for the commissions acknowledging that extensions of several deadlines set forth in the judgment had been discussed by the parties,4 and expressing a desire to consider them “as one package” because they were interrelated. The letter indicates that the DNR would not agree to join the commissions in petitioning the court for an eighteen month extension of the July 1, 1980, deadline for filing the initial reports because it feared a possible “domino *195effect” on other deadlines. It assured the commissions, however, that it did not intend to seek court enforcement of the initial deadline at that time. The letter clearly contemplates that continued negotiations concerning extension of other deadlines in the implementation of the pollution abatement program would follow.
The commissions’ initial motion to amend the judgment, filed on April 14, 1980, sought an eighteen month delay in the deadline to file the initial reports required by the judgment. On July 17, 1980, the commissions filed an amended motion to amend seeking still longer extensions of that deadline. No response by the DNR to either motion is of record.5 The Decade’s motion to intervene followed on July 28, 1980, only eleven days after the amended motion to amend. Neither the DNR nor the commissions contend that the delay between the motions to amend and the motion to intervene was excessive.
The trial court did not consider the Decade’s yearlong efforts to obtain information regarding proposed changes to the stipulated judgment. It did not consider that the Decade moved promptly to intervene once the commissions sought formal approval of the first of the proposed extensions of time to perform their obligations under that judgment. It focused solely on the time elapsed between the entry of the stipulated judgment and the motion to intervene. That focus excludes, as does the majority opinion, consideration of the fact that the motion was prompted by the commissions’ efforts to evade a significant term of the judgment, with the apparent blessing of the DNR, and that more such efforts were under negotiation.
*196In Natural Resources Defense Council, supra, the federal court of appeals found that the federal district court had abused its discretion in denying motions to intervene filed some three years after commencement of an environmental protection action, and after a settlement agreement had been filed, and held that the motions should have been granted. It stated:
Whether a motion to intervene is timely “is to be determined from all the circumstances.” More particularly, this court has noted:
[T] he amount of time which has elapsed since the litigation began is not in itself the determinative test of timeliness. Rather, the court should also look to the related circumstances, including the purpose for which intervention is sought. . . and the probability of prejudice to those already in the case. Hodgson v. United Mine Workers of America, [473 F.2d 118, 129 (D.C. Cir. 1972)].
Judged by these standards, it is clear that the District Court abused the “sound discretion” it is accorded to assess timeliness. In relying upon the age of the case and its closeness to settlement, the Court failed to take into account “the purpose for which intervention [was] sought,” namely, the concern for participation in the implementation of the agreement. [Footnote omitted; emphasis in original.]
561 F.2d at 907. The fact that judgment has been entered in this case does not diminish the force of the federal court’s reasoning. In this case, as in Natural Resources Defense Council and Hodgson, the intervenor does “not seek to reopen the settled issues in the case but [seeks] to participate in an upcoming, remedial phase of the litigation.” Natural Resources Defense Council, 561 F.2d at 908.
The commissions’ motions to amend had the effect of initiating new litigation between the parties. No proceedings on this litigation had even been scheduled at the time the Decade moved to intervene.
*197The trial court granted the Decade amicus curiae status with respect to all future proceedings, and directed the parties to keep the Decade apprised of all future negotiations towards a settlement. In doing so, the court evinced its recognition of the importance of citizen involvement in litigation affecting the environment. Cf. Wisconsin’s Environmental Decade, Inc. v. PSC, 69 Wis. 2d 1, 230 N.W.2d 243 (1975), (holding that this same public interest group had standing to assert the interest of its members in a clean environment under the Wisconsin Environmental Protection Act). Also implicit in this ruling is the trial court’s assessment of the “possible benefit to the court” to be derived from the Decade’s participation in the case — a factor considered by the courts in both Natural Resources Defense Council, 561 F.2d at 908, and Hodgson, 473 F.2d at 129 — and the trial court’s determination that the present parties would not be unduly prejudiced by that participation.
All of the above factors were appropriate and ripe for the trial court’s consideration in determining whether the motion to intervene was timely under the mandatory provisions of sec. 803.09(1), Stats. I am puzzled at the majority’s suggestion (majority opinion, p. 188) that the Decade should have introduced some type of additional “evidence” in support of these or other appropriate factors before the trial court was obliged to consider them. A resolution of the issue of timeliness in this case involves no factual determination. It involves an exercise of judgment in light of undisputed facts (filing dates, the stage of the proceedings) or of matters either ascertainable from the motion papers (purpose of intervention) or not readily susceptible to evidentiary proof (probable prejudice to existing parties6).
*198The trial court’s omission to consider any factor other than the age of the underlying action was a failure to consider “factors relevant to a discretionary determination.” Joint School v. Wisconsin Rapids Ed. Asso., 70 Wis. 2d 292, 309, 234 N.W.2d 289, 299 (1975). That failure constitutes an abuse of discretion. Wis. Asso. of Food Dealers v. City of Madison, 97 Wis. 2d 426, 434, 293 N.W.2d 540, 545 (1980).
In Wis. Asso. of Food Dealers, this court was directed by the supreme court to reverse and remand discretionary rulings of a trial court whenever it appears that the trial court’s analysis is incomplete because it has failed to consider factors appropriate to the exercise of discretion. See also Hartung v. Hartung, 102 Wis. 2d 58, 306 N.W.2d 16 (1981); United States Fire Ins. Co. v. E.D. Wesley Co., 100 Wis. 2d 59, 301 N.W.2d 271 (Ct. App. 1980).
It is not within the proper province of this court to find facts and weigh factors which the trial court should, but did not, evaluate. The question before us is not whether the Decade should be allowed to intervene in this lawsuit. The question on review is limited to whether the trial court properly exercised is discretion in determining that the motion to intervene was not timely under the mandatory intervention statute. In my view the majority usurps the trial court’s discretionary function by considering matters which it did not reach. These include such matters as whether the DNR can adequately represent the Decade’s interests, and the majority’s unexplained conclu*199sion that the Decade’s entry as a party would “add another layer and probably more delay to this suit.” (Majority opinion, p. 190). The latter finding is curious in light of the Decade’s purported purpose to avoid delay in implementing the judgment. In any event, such a finding is outside the realm of our review.
Wis. Asso. of Food Dealers gives this court no choice but to reverse and remand for the trial court’s complete analysis of the issue of timeliness under sec. 803.09 (1), Stats.
The motion to intervene asserts on behalf of the Decade’s members “an interest in protecting and preserving the water resources of the state . . . which are affected by the actions of the parties in this case, and in particular, by any proposed modifications to the previously court-approved stipulation entered into in this case in May 1977.” Although an affidavit and correspondence attached to the motion to intervene express the Decade’s dissatisfaction with the judgment itself, the record reflects that the focus of its concerns during the year preceding the motion to intervene was on the efforts of the commissions to modify the stipulated judgment (Record at 12, pages 4 and 6) and on “any current negotiations, or determinations not [to] commence enforcement action by the DNR, [which] could make even worse an already inadequate situation.” (Record at 12, page 10.)
Were the Decade to attempt to reopen the judgment after intervention, it would presumably be required to meet the tests for relief from judgment set forth in sec. 806.07, Stats., which specifies that the motion must be made “within a reasonable time,” among other requirements.
See note 1 supra.
In the first letter of record, dated June 22, 1979, the Decade indicated its awareness that the parties were negotiating unspecified changes in the judgment and stated:
As you are also aware, the Decade has a significant interest in this case because of what is undisputably a grossly overloaded and inadequate sewage system in Milwaukee and which is causing substantial injury to the interests which the Decade represents. Consequently, the Decade may wish to “participate” in the latest phase of the enforcement proceeding .... Whether or not we wish to “intervene” in the matter ... is a question which we can only answer after we have knowledge of what the propose [d] amendments to the stipulation state.
A subsequent letter dated July 25, 1979, expressed the Decade’s distress at “the present negotiations which are apparently going on in the backrooms between the regulator and regulatee [which] have not been brought out in the public arena . . . .”
See also note 1 supra, quoting the Decade’s letter of June 3, 1980.
These included the deadline specified for the completion of relief sewers, and those relating to expansion sewers and to the achievement of applicable water quality standards by the Combined Sewer Overflow abatement project.
A letter from counsel for the DNR to counsel for the Decade dated June 24, 1980, states that the DNR “notified the court” on April 29, 1980, that it opposed the initial motion to amend. No such notification appears of record, however, and no proceedings on either motion at which any such opposition could he expressed were ever held.
The federal circuit courts are split on the question whether prejudice to existing parties is an appropriate factor for consideration in interventions as of right. The 5th circuit court of appeals *198takes the position that this factor should he a consideration only-in permissive interventions under Fed R Civ P 24(h), which has its counterpart in sec. 803.09(2), Stats., and that it has no hearing in determinations under the mandatory provisions of Fed. R. Civ. P. 24(a), which is substantially identical to sec. 803.09(1). Stallworth v. Monsanto Co., 558 F.2d 267, 265 (5th Cir. 1977). But see Culbreath v. Dukakis, 630 F.2d 15, 21-22 (1st Cir. 1980).